SQL Agent Jobs Automatically Disabled After Modifying Maintenance Plan I modified a maintenance plan yesterday, to find out this morning (by using the last modified date from sysjobs) that all jobs related to the maintenance plan are disabled.

I can't figure out why this may be. The SQL Job Owner is sa and I merely changed a maintenance clean-up task with the maintenance plan and saved the job.
You haven't done anything wrong or gone crazy - it's a known bug and we've experienced the same thing. 

Our process was changed that we would script out job status and reapply after any maintenance plan work - until we could move off of maintenance plans entirely (which we did).  

Continuing on from the comments on the main question, there is an owner associated with the package itself which then gets copied over to the jobs and there's no UI to change it (you can modify it directly in the MSDB package tables though!) 

We remediated that later on but I am unaware if there's a link between that referenced login disappearing and the jobs being disabled. It's possible.