I don’t know if that is exactly a fear per say. I only say this because my boxers. Both are purebred, and I got both as puppies. But Tyson, my older dog, 3 yo. isn’t as likely to go through doors that aren’t open, or that he can’t open. I think he understands that he is supposed to stay where he is supposed to be if the door isn’t wide open. Then there is Marlee, 1 yo. who opens doors, and walks around like she is so cool, (lol.) It definitely may be related to a personality thing. Even half open doors Tyson gets anxious about going through them. Specially if he can’t push it open. I think of all things, be blessed that your pup, of all things he could hold onto, he is just unsure of doors. If you’re really concerned remember, patience, treats and lots of praise for when he goes through those doors! Good luck!