Answer
Check the Oracle Profile assigned to IFSAPP (typically IFS_INTERNAL) under Security > Oracle Users > Oracle Profile — IFSAPP is a regular Oracle database account like any other, so if that Oracle Profile has a password-expiration policy configured on it, IFSAPP itself is subject to expiring under that policy just like any other account assigned to the same profile. This is the first place to check if IFSAPP unexpectedly expires; it isn't something IFS treats as exempt from Oracle's own profile-level password-expiration settings.