Well, this is absolutely strange. As a crazy solution – does the watch “understand” that it is on your wrist. For example – Does AOD keep showing the watchface. If it is not, then this could be the cause – why it is not walking up on the notifications.
Also, note that if something coming between your watch and wrist. For example – tattoo on wrist. This may cause the phone to think it is not on the wrist.
Also, make sure that your Galaxy Watch4 Plugin has notifications Access permission granted. Many phone also like to revoke the permission often.
Follow this and see if this fixes your issue.