Currently, if you invite a person to an "Invite only" instance, the resulting invite link allows anyone to join that instance. It is not limited to the person that got invited.
This is causing issues for events that wants to protect themselves against malicious people raiding an instance. It is a reasonable request that an "invite only" instance only allows invited people to join, not anyone else.
To solve this issue, the invite link needs some kind of user-token so that it only works for the invited person. Or the server could have a list of invited people and requests from other clients which are not on that list are simply ignored.