السلام عليكم ورحمة الله وبركاته احسن الله اليك In lesson 21 of Manhaj As-Salikeen mentions that if we doubt whether we reached the Imam in Ruku or not, the Fatwa of Imam Ibn Baz is that you missed it and that you should make up the Rak'ah. But what if you have غلبة الظن that you made it?

Ustadh AbdulAziz al-Haqqan

This is the saying of the hanabilah that if he is in doubt then it doesn't count.