I don't have an 10 ultra.
On venus, while notifications somewhat worked in general, I had some disappearing notifications.
I use the Gmail app because the included one cannot handle my around 100GB main archive and it worked okish..
What I did to fix missing notifications here was to remove battery optimization from notification app itself.
Settings -> Privacy protection ->
Protection ( top right ) , scroll down to Special permisions -> Battery optimization. Default should show now not optimized apps, if don't see notifications there, click on that blueish Not optimized banner, select All apps, and search for notifications app. Click on it an turn off optimizations. You can try this way for other apps may have problems getting notifications.
You may need to reboot your phone.
I don't know if that works for your device but it worked for me.