If you see them only when wallet is in active use, those are local ones, trigerred by the app itself. Check if you see the device token under the push notification switch in the Settings.
 https://image.nostr.build/3fb0945edb7e0a37a1950150df6b4962e96f5650e9f2097f0e1951cbd0d765d1.jpg

For NWC to work when Minibits is killed or backgrounded, you'd need to be able to recieve them from the server, through firebase messaging - the same way push are delivered on incoming zaps no matter the state Minibits is in.

Most android devices are supported, but there are some that don't: i.e. if you use  a device that has no Google Play services installed (some chinese brands or privacy operating systems such as Graphene and similar). Those can not obtain the so called device token to which the messages are addressed.