Enterprise App on iOS 12 Won’t Verify
Solution 1:
The text actually does not state that it will be verified until it is deleted. Rather it says that the certificate will be trusted until the apps are deleted. That is different.
The signature on the app which is checked via the certificate (amongst other things) expires after a set time. You'll need to connect to the internet to verify the app again.
If that doesn't work, you'll need to contact your employer who published the app.