Universal ID is working on some instances but not others. The user gets the login invite but then when they try to sign in, they end up back in another instance without access to the new instance.
First, check whether or not Universal ID has been turned on for the instance. Universal ID is on by default for all new Marketo instances as of December 15, 2017 but if you're not sure, Support can double check. (For Support, this is located in Feature Manager and called "Single ID").
If Universal ID is confirmed to be on, then it may be a difference in password restrictions. If the new instance has different password security requirements than the other instances and the password doesn't meet those restrictions, you can end up in this situation.
Verify if the password settings match the other instances, and if not, then change the settings and send an invite again.