The way the cover edge is cut (in VVVVV shape), shows is not glued into the cup—it's a "press-fit" which means it is held in place between two plastic pieces inside the cup by pressure. There's no glue normally used for assembly.
I see NO trace of glue in your photos (which are very good, by the way).
The problem appears to be due to a fault in manufacturing. I think you have a "lemon."
They should be replaced by the manufacturer for free. When you return the set, tell them they "came apart." This is not an uncommon problem.
Otherwise they look in great condition which strengthens my opinion. There is no sign of mis-use or abuse.
Important: Find out before you attempt to repair them or the manufacturer could deny responsibility.