Updating column from another table - PostgreSQL 9.2
The statement looked perfectly fine in the first place. After excluding other options (like tables referenced without schema), it seems like the problem is caused by the 4 UPDATE triggers on the table (or one of them).

You should inspect the trigger code and understand what they are doing and which one of them is causing this behaviour. 

After that, it's really a business decision. If the triggers are not supposed to do this, you could decide to modify/correct the trigger code.

If they are doing what they are supposed to do, you might decide to not run the updates or - if these changes have to done absolutely - run the updates by disabling temporarily the triggers, bypassing the business rules.

You could disable one by one with:

ALTER TABLE public.ja_job_users 
    DISABLE TRIGGER trigger_name ;


or all of them with:

ALTER TABLE public.ja_job_users 
    DISABLE TRIGGER USER ;


which disables all user defined triggers (but not FK triggers), then run the wanted statements and then re-enable them (with the same syntax: ... ENABLE TRIGGER ...;. 

But I would strongly suggest to first inspect what these triggers do. It might be possible that only one is the root cause and the others are crucial and have to be kept enabled.

If the data are critical (can you afford to lose them?), I would also strongly suggest you take a backup now, before making any changes and do the actions manually, and inside a transaction. So, you can BEGIN; a transaction, run the updates, checking the data that have changed, and only when you are absolutely sure then COMMIT; the transaction.

Then take a backup immediately after the changes as well. This will allow later - and in your own time - to check thoroughly any side effect that this operation may have done by comparing the two backups.