Encrypted Push Notifications do not work at all in Android < 8 devices.
After doing some debugging, dpalou noticed that seems to be a problem with lazysodium that has been fixed in version 5.1.1. The problem is that version has not being released yet so it is not available via Maven
Dani already reached the author of the plugin about the 5.1.1 not being available trying to find a workaround with no success https://github.com/terl/lazysodium-android/issues/58
For now, we added a mention in Moodle settings that this feature only works in Android for versions >=8 but it would be great to upgrade the library to support older devices.