I use addy.io for email forwarding for privacy reason. Used it before switching to proton so I still don't use simplelogin.
When I get a calender invite, if I click "Show in Proton Calender" on my email, I get a popup that says "You email is not on the participant list" because it is not my protonmail address. I am forced to download the ics file and then import it. It would be nice if there was an option to bypass this warning and import the event in one click