Looking at the iOS platform security document ([0] page 68), apps use NSFileProtectionCompleteUntilFirstUserAuthentication by default, which keeps keys in memory after the first unlock, regardless of the Power + Volume Down lock. If an app opts into NSFileProtectionComplete, I believe the keys are purged from memory upon locking.
[0] https://manuals.info.apple.com/MANUALS/1000/MA1902/en_US/app...