Edited iCal events do not always save

I double click an existing iCal event to see more details.

I edit some text in the Notes section of the event.

I click off of the event.

I then double click the event again - and sometimes the edits I've made to the text have not saved. This has only recently started happening, and it doesn't happen every time I edit an event. Maybe 50% of the time it doesn't save.

Sequoia 15.2

Macbook Air M1 2020