I encountered this problem with a kMail identity with alias addresses.

In my concrete case, a distribution / mailing list address was defined as an alias address for my personal address in kMail settings -> identities -> Edit -> eMail aliases.

I entered a new incident and invited this distribution list (in this case all@example.com) and confirmed the editor.

The invitations were sent correctly, but the indicent was not visible in my own (!) calendar. I always use the month view, but I don't expect it's specific to this.

Only after removing the mail alias from my mail identity - which caused kOrganizer to crash, btw. - the incident automagically appeared in the month view.

Reproducible: Always