السلام عليكم و رحمة الله و بركاته I follow the opinion you mentioned, that the Ma‘moon has to recite behind the Imam in both silent and loud prayers. Because it is new for me to recite in loud prayers I sometimes forget to recite after the Imam, so is my Salah valid because of forgetfulness.

Ustadh AbdulAziz al-Haqqan
Valid.