If you have the cone pointing to tabbed enemy on then the gunsight disappears, sort of, it come son when enemy plane is in your sight but then disappears when it isn't.

This bug has been noticed once during development, but I cannot reproduce it on my end.

Which aircraft did you fly? Which one was the enemy?
Did you have any cockpit damage? Was it daytime or night with any searchlights visible?

Hurricane flying. Seems to be any plane locked on too.