Thanks a lot for reporting this. It should hopefully be fixed in version 7.2.1.6, which I’ve just released.
I introduced a PHP session in MT beta version 7.2.1.4 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.
The latest version uses cookies, which is the recommended alternative.
I hope that helps. Please let me know if you still need assistance.