This is a very annoying known bug (one time I was busy rebooting my phone for about half an hour because it kept running into the same bug), but it’s not a bug with the encryption. It’s a bug with the boot process not waiting until a specific service is started, so sometimes the service is ready and the unlocking works, but sometimes the service isn’t ready and the unlocking doesn’t work. You can tell it’s the boot process itself, because on a ‘correct’ boot, it takes some time for the unlock screen to pop up, whereas on a ‘bad’ boot, the unlock screen shows up almost instantly after the Jolla/Sailfish splash screen. Having an encrypted device makes the bug more obvious, but the encryption itself isn’t at fault.