I've found a SQL Job(s) on a server which run an SSIS package, I've changed the 'run as' drop down to a proxy account which has permissions to run the package. I will change it to my proxy account, run it a few times. However, when I come back to the server after a while and I'll find that it has reverted from my 'proxy account' to the 'SQL Server Agent Service Account'.
Please let me know if you have any suggestions as to why the identity is changing or where to look for clues as I've not seen anything in the logs which indicate that the identity was changed by the system.