The mud capped end is missing, what could be happening?
There could be a number of reasons:
If the center of the hole seems to be missing, the mud might have shrunk as it dried and the center part fell out. Low impact
If early spring, a bee might have exited the hole. They don’t always remove all of the mud capped ends. No impact.
A predator might have gotten in like a carpet beetle. You’ll find this out in the fall during harvesting. High impact.
If all of the mud is missing, it’s quite possible a bird was looking for a tasty snack. High impact.
If you are concerned, remove the holes from your yard and place them in a BeeGuard bag. Store the bag in an ambient temperature, outdoor protected place like a shed or unheated garage.