Hey @nathank, thanks for the report!
It’s indeed a regression, you are also using the include_expired_events_on_calendar site setting, right?
The issue is that we were creating an outline of the background color for expired events, but events in PMs don’t have a background color set at all.
This PR should fix it:
https://github.com/discourse/discourse-calendar/pull/525
It also replaces the outline style for expired events with a simple opacity.