Calendar updates lost if changing Calendar and other details

From my MacBook Pro, using OS X El Capitan v 10.11.3, using Calendar version 8.0:

I have a few different "calendars" set-up, the calendars that are identified by a color swatch. All the calendars I'm using are iCloud calendars.

What I've done that causes the problem, but I've been doing it for ages and it used to work:

0. starting condition: I have an event in the calendar I happen to call "FYI" meaning tentative.

I decide to update the event because I want to "ink it in".

1. I change the calendar to my main calendar

2. I add some more text about the event

3. I might change the time of the event.

Everything looks good momentarily.

Problem: at some point the text and time updates disappear. The assigned calendar change (and its color) stick, but all other updates are lost.

This has been happening for about a month.