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 email@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.
Addendum: This can easily cause you to lose track of incidents / appointments you created yourself... And that might be anything from embarrassing to annoying or worse...
Probably a duplicate of Bug 352214 (https://bugs.kde.org/show_bug.cgi?id=352214) (or the other way around?)
*** This bug has been marked as a duplicate of bug 352214 ***