Are my previous prayers invalid because I used to say in prostration "Subhana Rabbi al-A'la" (Glory be to my Lord, the Most High) with a fatha over the yaa in "Rabbi" instead of a kasra, even though I know now that it should have a kasra, given that I had doubts about how to pronounce it previously but did not ask?

Opening or silencing the yaa’ of the first-person singular pronoun (mutakallim) in a worshipper’s recitation of “Subhana Rabbiy al-A’la” (Glory be to my Lord, the Most High) does not affect the validity of the prayer. This is because both pronunciations are linguistically and recitationally correct. The verse, “My Lord is He who gives life and causes death,” (Rabbiya alladhi yuhyi wa yumitu) has been recited with both. In these instances, the yaa’ can be pronounced with a fatha (opened) or omitted phonetically when connecting words, due to the meeting of two quiescent letters.