In this article I will share my short analysis on this question
Shaykh al-Mufīd’s (d. 413 AH) doubt in the miscarriage first appears in his book Al-Irshād which is a book regarding the biographies of the infallibles. In the chapter regarding the children of Imam ‘Ali (as) he mentions:
وفي الشيعة من يذكر أن فاطمة صلوات الله عليها أسقطت بعد النبي صلى الله عليه وآله ولدا ذكرا كان سماه رسول الله عليه السلام – وهو حمل – محسنا فعلى قول هذه الطائفة أولاد أمير المؤمنين عليه السلام ثمانية وعشرون، والله أعلم
Among the Shīʿa, there are those who mention that Fāṭima, the blessing of Allāh be on her, after the Prophet had a miscarriage with a son, whom the Prophet, may Allāh bless Him and His Family, had (already) named during her pregnancy as Muḥsin. According to this group there were twenty-eight children of the Commander of the Faithful, the blessing and peace of Allāh be on him. Allāh knows and judges best.Al-Irshād, vol 1, pg 355
Now some have objected that this statement is not conclusive in pointing out the opinion of al-Mufīd himself.
They mistakenly assume that this conclusion comes from al-Mufīd’s mention of “Among the Shīʿa”, perhaps because of their copying this argument from the book Maʾsāt al-Zahrā’ by Sayed Jaʿfar Murtaḍā. 
But the argument is based on what is mentioned at the beginning of the chapter by Shaykh al-Mufīd:
باب ذكر أولاد أمير المؤمنين عليه السلام وعددهم وأسمائهم ومختصر من أخبارهم فأولاد أمير المؤمنين صلوات الله عليه سبعة وعشرون ولدا ذكرا وأنثى: الحسن والحسين وزينب الكبرى وزينب الصغرى […]
(This is) an account of the children of the Commander of the Faithful, peace be on him, their number and names, and a selection of reports about them. The Commander of the Faithful, peace be on him, had twenty-seven children, male and female: 1. Al-Ḥasan 2. Al-Ḥusayn 3. Zaynab the elder 4. Zaynab the younger […]Al-Irshād, vol 1, pg 354
Shaykh al-Mufīd mentions that the Imam had 27 children, and proceeds to list out all their names, there is no mention of Muḥsin, till he proceeds to name another group among the Shīʿa which based on their view, the children would be 28. It was already clear from the beginning of the chapter that al-Mufīd does not count himself among those.
Some may suggest that al-Mufīd affirms the existence of a child named Muḥsin in another book Al-Ikhtiṣāṣ, but according to experts in the field, this book was not authored by Shaykh al-Mufīd and was misattributed to him. I have written about this more in another article: Who is the author of Al-Ikhtiṣāṣ?
Consensus of the Shīʿa
It has also been mentioned that Shaykh al-Ṭūsī (d. 460 AH) has claimed a consensus on the matter between the Shi’a this is based on the following statement:
ومما أنكر عليه: ضربهم لفاطمة (ع)، وقد روي: أنهم ضربوها بالسياط، والمشهور الذي لا خلاف فيه بين الشيعة: أن عمر ضرب على بطنها حتى أسقطت، فسمي السقط (محسنا). والرواية بذلك مشهورة عندهم. وما أرادوا من إحراق البيت عليها – حين التجأ إليها قوم، وامتنعوا من بيعته.
And what he was criticised for is their beating Fatima (sa) who is reported as having been beaten with whips. What is famous and what is not disputed among the Shīʿas is that ‘Umar (ibn al-Khattab) hit her on her stomach till she miscarried, and the child whom she miscarried was named Muḥsin. Such a narrative is quite famous among them. Add to this their desire to set her house to fire when people sought refuge with her and refused to swear the oath of allegiance to him (to Abu Bakr).Talkhīṣ al-Shāfī, vol 3, pg 156
The question then arises, is it possible for Shaykh al-Ṭūsī to claim consensus for something his own teacher opposes? The answer is yes and this has happened multiple times and is known by the scholars.
Perhaps the reason for al-Ṭūsī’s wording in this case is the polemical nature of this work, Talkhīṣ al-Shāfī is a summary of al-Shāfī fī al-imāmah by his teacher al-Sharīf al-Murtaḍā (d. 436 AH). al-Murtaḍā had authored this book to refute a book authored by a Muʿtazili scholar. In refuting ones opponents it is not untypical that one may refrain from mentioning that there is a difference of opinion among their sect.
Another possibility is that al-Ṭūsī meant that there is an agreement on the general event rather than the specific details he mentions like the miscarriage of Muḥsin.
The above quoted from Al-Irshād is already one example of a contradiction if the aforementioned attempt at reconciliation is not accepted. But there are numerous other examples where a contradiction is found with his teachers, or even against his own proclamation of consensus.
Shaykh Yusūf al-Baḥrānī (d. 1772 CE) mentions in this regard:
قد كان عندي رسالة لشيخنا الشهيد الثاني (ره) قد تصدى فيها بنقل جملة من المسائل التي ناقض الشيخ فيها نفسه بدعوى الإجماع على الحكم في موضع ثم يدعيه على خلافه في آخر وفيها ما ينيف على سبعين مسألة
I had a treatise written by our shaykh al-Shahīd al-Thānī (rah) in which he addressed a number of issues in which Al-Shaykh [al-Ṭūsī] contradicted himself, claiming consensus on the ruling in one place, then claiming his disagreement in another, and the treatise exceeds more than seventy issues.al-Ḥadāʾiq al-Nāḍira, vol 5, pg 30
One of the most popular examples is Al-Ṭūsī claiming a consensus on the probative force of a solitary report (khabar al-wāḥid)  while his teacher al-Sharīf al-Murtaḍā famously rejects the probative force of khabar al-wāḥid. 
There is no conclusive evidence that Shaykh al-Mufīd believed in the miscarriage of a child named Muḥsin. It should be noted that this specific detail being rejected does not necessitate that someone rejects the whole event of ʿUmar coming to Sayeda Fāṭima’s house threatening to burn it, nor does affirming the miscarriage of Muḥsin mean that someone believes in the event mentioned, as even some Sunni scholars have mentioned this. 
|↑1||See Maʾsāt al-Zahrā’, vol 1, pg 167|
|↑2||ʿUddat al-Uṣūl, vol 1, pg 126|
|↑3||Rasā’il al-Murtaḍā, vol 1, pg 26|
|↑4||Tārīkh al-Ya‘qūbī, vol 2, pg 213; Tārīkh al-Ṭabarī, vol 5, pg 153; Tārīkh by Ibn al-Athīr, vol 3, pg 397; Ansāb al-ashrāf of al-Balādhurī, vol 2, pg 189; Al-Iṣābah by Ibn Ḥajar, vol 3, pg 471; Lisān al-Mīzān of al-Dhahabī, pg 268; Mīzān al-i‘tidāl, vol 1, pg 139; al-Qāmūs al-muḥīṭ of Fīrūzābādī, vol 2, pg 55, and other various sources.|