Teracube 2e (2020), Android 10, May 26 2022 build 1.0-q-20220526188859-stable-2e
Was having this same issue for a while…
Upon looking around, I spotted this post and it seems that you have to let your phone be registered with Google (via microG settings) in order to get notifications for some apps. I had turned that feature off initially because I wanted to be as far away from Google as possible, but after turning it back on I was able to get notifications instantly. I also turned battery manager off to see if that was the issue, but it seemed to have no effect.
Could this be the fix for y’all?