When you execute "/locate EndCity" while standing in the Main Island, it returns an invalid position in the Void, with X Z coordinates less that +-1000. There won't be any End City in that position obviously.
MC-141192
/locate EndCity in Main End Island not working
Resolved
Duplicate
0
FACS01
(Unassigned)
Unconfirmed
Minecraft 18w50a
Retrieved
Related slightly, while flying through the void I got the advancement for entering an end city. Obviously, there was nothing there.