An Unanswered Curiosity Regarding Shaykh Tusi

Was recently reading something related to Shaykh Tusi (d. 460/1067), and it reminded me of a point about his biography that I have not been able to reach a convincing conclusion and explanation for. In some of the Sunni bibliographical works, it says that he grew up as a Shafi’i Sunni, specializing in Shafi’i jurisprudence, but that when he arrived in Baghdad in 408 AH, he attended the classes of Shaykh Mufid, got exposed to Imami theology and legal theory, which eventually led to his conversion.

See for example: Al-Dhahabī’s (d. 748/1348) work Siyar A‘lām al-Nubalā, al-Wāfī bil Wafāyāt by al-Ṣafadī (d. 764/1363), and Tāj al-Dīn al-Subkī’s (d. 771/1370) Ṭabaqāt al-Shāfiʻiyyah – a biographical dictionary of the scholars of the Shāfi’ī legal school. Interestingly, all three works are from the Mamluk era.

The rest of the details written in these entries about the Shaykh are more or less accurate and true. However, the point about him being a Shafi’i is not mentioned in any Shi’i work at all. A few researchers have tried to address this point in their introductions to some of the works of Shaykh such as his al-Khilāf or al-Rasā’il al-‘Ashar saying this is a false claim regarding Shaykh, but do not seem to give any explanation for why these bibliographers could have gotten something so blatantly wrong or what motive would they have had to fabricate something like this. I inquired about this from both Shaykh Yusafi Gharavi and Sayyid Ahmad Madadi while in Qom around 2015/2016 and they did not seem to take these entries too seriously; both said that if this was the case, our bibliographers would have mentioned it too as it would be a favorable point for the Shi’a.

Did these Sunni bibliographers make something up, if so, what would be the motive for that? Was it just an honest error, and if so, what led to it? Perhaps it was the works of jurisprudence written by Shaykh Tusi where he clearly shows a lot more attention towards Shafi’i opinions and is to an extent even influenced by the structure of their works; did that become a reason to formulate this false perception? Or did Shi’i bibliographers simply not mention it, perhaps deeming this point irrelevant, or were they unaware of this matter? Not sure how much it would really matter if it were true that Shaykh was a Shafi’i before moving to Baghdad, but for me, it remains an unanswered curiosity.