That's not really true at all - you can of course unlock your iPhone without entering PIN for every screen lock which should give you a clue that keys for disk encryption generally aren't purged when iPhone is locked.
Some keys are, but not the ones that are the issue here.
I've even seen conditions where iOS devices reboot and still retain keys.
If you unlock the screen using Face ID the OS gets the keys from the Secure Enclave which, depending on the model, does the face recognition itself or using the normal processor in some kind of secure way. Just like if you unlock the phone using the pin code, the OS gets the key from the Secure Enclave which makes sure it’s not easy to brute force. The PIN code is not the key itself of course.
The only key that sometimes gets retained at reboot is the SIM unlock.
From what I gather the more secured keys should be discarded 10 seconds after lock screen event. Lower security keys stay in memory to allow background activity.
Encryption on ios, if i understand correctly, is on a per file basis. There is thus no "mount" event to look for and it should provide no value to use a less secured key if you do not intend to run on background because decryption is supposed to happen on the fly.
PS: Also if I remember correctly pressing down the emergency sequence (holding power + volume up) discard ALL keys instantly and unlock require the passphrase as if you just rebooted. Emergency call don't need to be issued just initiated (must hold 10 sec or confirm on screen to make the actual emergency call).
Some keys are, but not the ones that are the issue here.
I've even seen conditions where iOS devices reboot and still retain keys.