What are the implications of returning to prostration after having started reciting Al-Fatiha out of forgetfulness and then remembering this during pro prostration; is it obligatory to restart the prayer or to continue it, and what should have been done in this case?

If someone forgets a pillar, like prostration, and it is possible to rectify it, they should perform it and whatever comes after it. However, if rectification is impossible, that rak'ah (unit of prayer) is nullified, and they must perform another rak'ah in its place and perform the prostration of forgetfulness (sujud al-sahw).

If they remember it after beginning the recitation in another rak'ah, the rak'ah in which the pillar was omitted becomes void, meaning it is disregarded, and the subsequent rak'ah takes its place.

Sheikh Ibn Uthaymin was of the opinion that a rak'ah is not invalidated unless the worshiper reaches the point of the omitted pillar in the subsequent rak'ah. Accordingly, they must go back unless they have reached its position in the second rak'ah. If the worshiper reaches its position in the second rak'ah, then the second rak'ah becomes the first.