There were quite some bugs solved in 9.12.x. Also, when you migrated from <9.12.0 you should look here: https://docs.mendix.com/refguide/scheduled-events/#migration to determine if you need to change settings manually that could not be converted automatically.
Go Make It.
No mention of any problem or fix of this kind in the release notes of 9.12. Another step is checking your logfiles. And note that starting the scheduled events is done by the MBS.
Ah, never mind. See Marco’s comment.
It may be a Mendix issue, 9.12 is a recent version. Reach out to the Mendix support team for this kind of issue.