I get emails with .ics attachments. I have to open the email... click on the attachment and only then I can accept or decline the invite.
I used to just right-click on that email and click accept/decline.
These invites are not recognized as calendar events. They are treated as email with attachments.
There used to be a different icon on the left of the email that showed it as an invite for the calendar.
Now it it just email with a paper-clip icon.
Remove "AT" and replace with @