On Android? It must be an app-specific issue because it's possible for apps to implement alarms so that they work before unlocking the device after a reboot, but I don't know the technical details behind it.
I've had that happen a few times and the alarms went off on time but they used the default alarm tune instead of the one I had selected, presumably that data was still encrypted.
Android actually have a special mode for apps that needs to work after a reboot (e.g.: while the FS encryption is still locked), and Alarms is one of the apps that uses this special mode. I don't remember how it is called though so sorry for the lack of sources, you will have to trust me on this one.
The amount of misinformation in this thread is huge though. I generally read every changelog for major Android updates and the amount of engineering going on is amazing. People just assumes that a few things doesn't work while they do.
Unless of course OP is using a custom alarm app this may be true, but then it is the app fault and not Android.
I've had that happen a few times and the alarms went off on time but they used the default alarm tune instead of the one I had selected, presumably that data was still encrypted.