Thanks a lot for reporting this. It should hopefully be fixed in version 18.104.22.168, which I’ve just released.
I introduced a PHP session in MT beta version 22.214.171.124 as a last minute fix for another bug I noticed during my testing. I’ve never tried to use PHP sessions in a WordPress plugin before, and unfortunately hadn’t read up on the issues it can cause – such as problems with the REST API.
I hope that helps. Please let me know if you still need assistance.