Dismantle the station he is returning to and any other work station he is associated with (edit: at that same location).
That will keep him where you want him to stay.
Are you setting him on guard in Siptah before assigning to station? Does your base on Siptah have all needs?
I had kind of the same problem yesterday with Maev the Magnificent. I placed Master Blacksmith Kiris in my jungle (main) base but he did not show up in the garrison workbench. At first I pulled Anora Steelfingers out of the base and portalled her to a portal hub I have at the mounds and told her to stand guard. Went back to my base picked up Maev and ported to the same portal and told her to stand guard. She did not, she took of and went into returning home immediatly and walked away from my hub. I made sure she could not run into trouble then set her on stand guard again. I ported back to check if Kiris was in the bench but he was not, Maev was and when I looked around she was sitting on a bench near the barkeep. She portalled back as soon as I did.
Today however after the daily reset on my official server Kiris is shown on my blacksmith bench and Maev is not running home when I tell her to stand guard at my portal hub.
Is there anything in my story that rings a bell for you regarding your problem?