Appointment invitations from google appear as
html page which then contain the buttons to accept or decline
Invitations from outlook are displayed in the mail client with an empty body and without attachments. In rainloop the mail contains an invite,.ics file.
The interaction between the calendar and the mail apps does not harmonize yet.
So you can not really use it!
And please now no statements about the fact that then the other providers do not adhere to standards.
There must be a solution for this…