Mailing list trigger with delay not working after December 1, 2019

Hi all! Anyone has issues with mailing list triggers with delay?
We have a script running which adds contacts to the mailing list. On Mailing list trigger we have settings that a survey will be sent to the contacts added to the mailing list after 7 days.
All was working perfectly fine until November 30th 2019. After that the contacts are still created (so the script is fine), but the surveys are not sent out. As a troubleshooting we created two mailing lists: one with 1 hour delay and another one with immediate distribution on contact addition. I manually added contacts to both. Only one email (the one without delay) was received.
Support has been investigating it for some time and still nothing works, so I am here for the power of the Community.
Any ideas what happened? I suspect something to do with the XMD updates to be honest.