Unfortunately, the fix (clearing the Android System WebView cache) only lasts for a few hours. After the phone stays idle or goes into deep sleep overnight, both the 30-second loading lag and the total 2FA notification lockout return.
It seems like a recurring state corruption in the Android 16 firmware on the FP6. Clearing the cache is just a temporary “band-aid”, not a permanent solution. The system seems to “forget” the registered notification channels again after some time.
I’ve reported this as a persistent bug to Fairphone support.