I had also had this issue today on a test server (e.g., desktop). I've been defecting and re-mastering a lot. I changed my service account today as well - using my first MSA. Someplace one of my jobs (1 of about 60) got stuck as neither completely master or completely local. I could not save changes to it. I could not script it out - got the same error as you did.

I updated the system table for the job to remove the multi-server category. (In the past, I hacked a target job for fun to change it to a local job. I knew what to look for.)

update msdb.dbo.sysjobs

set category_id = 3

where name = 'MyJobNameWithTheIssue'

It was not enough because it had a related sysjobservers record with a non-zero server_id.

DELETE FROM js

FROM msdb.dbo.sysjobservers js

INNER JOIN msdb.dbo.sysjobs j

ON js.job_id = j.job_id

WHERE j.name = 'MyJobNameWithTheIssue'

This was enough. It became a local job that I could edit and script out. (I am very glad I did not have to cut and paste each step.)

PS There is a bug that will not let you remove the default instance as a target from a named instance master on the same machine. (I submitted a bug for this recently. The code is trying to insure that a target is not the master before deleting the job. Good thing to do after a host rename, but it fails to account for named instances.) I had to put the master on the default for testing. Perhaps the issue with this job is related to the bug.