S4 mini forget encryption pin topic

Hi everybody,

I have got a couple of S4 mini, which are encrypted and saved via a 4 digit PIN.
Three of these devices had the problem, that they can´t remember the correct PIN to decrypt the device after a power down.

So I had to wipe all three devices and installed them again.

Anybody heared something about this problem? Is anybody knowing a specal reason for this behaviour?

Thanks in advice
Temporer