I've run into this on 5.9.1 and 5.10 (on Neon), and on Fedora 27 some months back, but just now saw what triggers it.

If I add the Google calendar using the appropriate resource for my company's email (it's Google Apps), I'll see all pending unconfirmed invitations on my calendar. This is working fine on my personal computer that doesn't have my work email on it. When I set up my email on my work computer, even the round about manual way, the second the same email address my calendar uses goes into an identity in KMail, all open invitations disappear; only confirmed items remain. Keeping the identity config window in front of korganizer, I saw this was instantaneous.

I've tried to trick it. In General settings on korganizer I deselected to use default email and left a noreply in. Still, as soon as that associated email address is in an identity, it all goes away.

If anyone knows any hackery to keep these things separate for now, that would be great, I really like Kontact and would prefer to keep using it at work. Not sure if a bug is open for this; googling hasn't helped much, but I suppose that avenue is rather ironic in this situation.

Thanks!
-Blake