Same situation here. Used smart switch from my s7 edge to s9+. I tried everything here to no avail. Uninstalled smart switch, restarted phone, and app badges are sporadically working (picks and chooses when it wants to be accurate/work). Incoming text preview still not working. Will not be using any type of backup/cloning software to avoid this issue in the future.
The following worked for me :
Open settings, open apps. , hit the 3 dot menu, hit spec. access, hit Samsung experience, turn the toggle to on.
Again - that only adds badges but doesn't fix the unread message count in the badges. Somehow I think this is a Google problem but it doesn't hurt to pressure Samsung as a customer. They pull more weight with Google than we can.