Does excessive whispering of the letter "tāʼ" - with others distinguishing it - affect the validity of reciting Al-Fatihah, or is it considered a hidden لحن (لحن خفي)?

What the questioner mentioned regarding the issue of whispering does not invalidate the prayer. This is because not perfecting the characteristics of letters, as known in the science of Tajweed, does not invalidate the prayer; rather, it is considered a subtle linguistic error (Lahn Khafi). Furthermore, obsessive thoughts (waswasa) concerning the articulation points of letters are among the stratagems of Satan. Therefore, one should strive to overcome these thoughts, seek refuge in Allah from Satan, and disregard these whisperings.