EDIT (May 13, 2019, 13:50 PM GMT+0): The issue will be fixed in version 21.0.1 and the update has been released for Android now. Please update your game!
There is currently a known issue with the Friend Invites where some invitees are currently not recognized. We hope to release an update with a fix for this issue as soon as possible.
Our apologies for any inconvenience caused by this issue!