SQL Server 2012 Job Seemingly not running at all, manual or scheduled, no logs

Question

We recently installed a side-by-side instance of SQL Server 2017 on our DEV environment, and almost all of our DBs and jobs have migrated over. However due to compatibility issue, we still have a couple of jobs that need to be run on the 2012 instance. It appears they have not been succeeding in some time, however.

When I run them manually, either right-click on the job, or through job activity monitor, or via T-SQL, it "starts" normally but there's no actual activity. It doesn't show as running in the activity monitor, the tables don't get affected at all, nothing shows in the history for the job. I've tried a few different jobs and they all exhibit this "phantom" behavior.

I've checked the system table for SQL Agent Job history and it also does not show any new activity since over a month ago (roughly when the migration happened). I've looked at the various logs in MSSQL11.MSSQLSERVER\MSSQL\Log directory and they show nothing at all when I'm trying to run these jobs.

I've tried changing what account the SQL Agent runs under to no affect, the entire server restarts every week but I've also restarted the components of it myself and while that shows normally in the above log file, there's nothing I can do that seems to generate real activity in the logs or jobs.

As far as I recall from the installation, none of the directories for the 2012 instance were modified at all, we simply detached and reattached the user DBs to 2017. The 2012 temp db, installation location, log files, system DBs, etc. all were unchanged during the process.

I have not been able to find anything about this in my searches all day, and I'm pretty much out of ideas. I could try to uninstall / reinstall the 2012 instance but I'd be worried about disrupting the 2017 instance even though it should be fine, as theoretically the 2012 instance should have been fine. Any suggestions would be greatly appreciated.