I’ve gone through all my addons, disabling one-by-one. The only one that makes the “?” re-appear is by disabling AMR.
This survives /reload, logout/login and restarting WoW for me.
As I said, I have disabled all other addons and using the “Vanilla” UI.
I’ve also re-installed the addon with no change.
That is weird… as far as I know, our addon has no interaction with the calendar, doesn’t call any methods on the calendar, etc. So not quite sure how that would happen…
Yep, I kinda guessed that. Willing to help troubleshoot, but I really don’t know where to begin.
FYI, I did a “scan” using the built-in Blizzard tool as well as I deleted and let WoW recreate the Blizzard_Calendar directory under AddOns neither of which changed the behavior.
So, further troubleshooting, I completely wiped my WTF and Addons directories.
The invite “?” is still being suppressed. So I guess its not directly related to AMR addon.
I installed AMR addon, still suppressed, I again disabled AMR addon, but this time the suppression persisted.
None of my friends are seeing any of this behavior. I’ll let you guys off the hook! 
Sorry I wasn’t much help on this one… it’s actually really easy for errors in one addon to bleed into another… Blizzard went with a weird design for addons that lets them mess with each other, making it really hard to track down the original source of errors sometimes.