When inviting around 10 people to my realm, many of them received the invitation pop-up on the menu screen.  But when they opened the realms menu, the "piece of paper" that lets you accept invites did not show up.  This only seemed to happen to people who had not used the realms free trial yet.

Tried relaunching and resending invites.  But nothing worked.

I am 100% sure I typed the usernames correctly, as their skin appeared in the pending members list.