I haven't received a notification from Thunder in a few weeks. I double-checked the settings to make sure "Enable Inbox Notifications" is enabled, and it is.
Anyone else have issues with notifications recently?
Aside from everyone else's suggestion to disable battery optimizations, which is a likely culprit, I would also suggest logging out of your account(s) and back in. Sometimes if there is stale info, the notifications will fail silently in the background.