Unable to invite again an unbanned user

[giomfo]

This issue is observed when the banned/unbanned user is hosted on another instance (federated homeserver).

To reproduce:
1- Create a room on HS1 with user1
2- Invite user2 hosted on HS2
3-Join the room with user2
4- user1 bans user2, then unban user2 and invite user2 again
5- user2 receives the invite, but they are not allowed to join. The alert is: "Cannot join user who was banned"

[giomfo]

this is a known problem matrix-org/synapse#1563
This is a server (synapse) issue