Hi there, if I understand right you're trying to use an xpath in a offline mode?
From the documentation:
https://docs.mendix.com/refguide/offline
4.2 XPath
The database used to store data on your mobile device for offline use does not support complex queries. As such, the XPath setting is disallowed on all the widgets accessible through the offline device profile. Alternatively, the simple constraints found in the database data source can be used, as well as modeling complex queries using entity access.
EDIT -------------------------------------
sorry I read your question wrong, apparently you already used an xpath in the entity access as a access rule. but you're right it does seem to say xpath in entity access should work...
I am not sure if system.owner works with offline apps, but try using the solution I gave as a comment in this post:
https://forum.mendix.com/link/questions/87339
If system.owner does not work, than the object will never be shown in the mobile app. You have to associate it to the account object.