Outlook (M365 desktop client) Calendar turns red. Alerting of bad entries?

The company I work for is currently undergoing a merger & acquisition.  My current company and new company both use Exchange Online.  My Outlook client is maintaining both mailboxes, which includes the Calendars. They are being viewed side by side.  Or sometimes in Overlay.  (It does not appear to change what I am experiencing.)
I have forwarding turned on via OWA options so that all email goes to the new parent company email address.  This includes meeting invites.  Both of my mailboxes have Online Archive enabled.  (I don't know if this is a relevant data point.)
Sometimes when trying to manually sync the calendars (i.e., have the same events showing in each calendar), my current company calendar turns shades of red.  As if alerting me to a conflict or something else Outlook can't immediately resolve.
The calendar will usually turn back to it's normal color when I act on a particular meeting invite.  Or sometimes, I think it just clears itself.  I'm lost as to what the trigger is in Outlook desktop client that causes the entire calendar to turn red.  I can't find any documentation on that.