Why does my Encrypted HTC One require storage decryption on resume?

A

AC Question

Encrypted HTC One requires storage decryption on resume

My HTC One seemed to work correctly.

I encrypted the phone which seemed to be successful.

My newly-encrypted HTC One now seems to require a cold start after being left unused for a relatively short period of time. For 20-30 minutes after locking it it behaves as expected and presents the normal login screen which I can successfully use to gain access. After a longer period of time the phone presents the storage decryption dialog box and proceeds to boot after I supply that password.

Either the phone is re-booting itself for mysterious reasons (hardware fault?) and therefore requiring the decryption password as part of the startup.

OR the phone is un-mounting the encrypted filesystems it enters a sleep/power-saving mode.

As it is the phone is pretty much useless and I'll need to reset it but have I missed something?

Is this a known bug? I've searched extensively and haven't found reports of problems quite like this (which has got me thinking of hardware faults).