iOS Calendar app shows "!" next to all calendars, but they sync OK

The Calendar apps on my iPhone and iPad generally work fine - events from subscribed calendars are synchronized across all of my Apple devices Just Fine.

HOWEVER, on both my iPhone and iPad, the list of calendars (viewed by tapping "Calendars" from the main app screen) shows the red, circled exclamation point next to all of my iCloud subscribed calendars. (Exceptions: the Birthdays and Siri Found in Apps calendars do not have the exclamation point, but neither are listed as iCloud calendars.)

I have tried (1) quitting and restarting Calendars, (2) re-booting the phone (including "hard" reset), and (3) signing out of and back into my iCloud account on the iPhone. No change to the exclamation point status.

This has been going on for at least a week now (that's when I first noticed it, but admittedly the problem could have started before then).

I do not see any error conditions in my iCloud account under either iOS or MacOS.

Both the iPhone and iPad are up-to-date with respect to system software.

Admittedly, I could just continue on my merry way, given that events do seem to sync just fine, but I'm never comfortable living with apparent error conditions on an ongoing basis.

I'm stumped! Any constructive suggestions???