That is, the "sharing" options are completely missing.
I had 1 calendar of several like this. I never could figure out how to fix it; I had to create a new one, move events from the broken one to the new one, and delete the broken one.
Yes, it's a pain, but there wasn't anything else for it.
Note that - depending on your calendar app - you may have to go into your phone Calendar app settings to make the new calendar actually visible!
