Subform lookup field not populated when offline

Subform lookup field not populated when offline

Lookup fields on the parent form work when offline, but any lookup fields that are in a subform always show the "no internet connection" screen when offline - even if you previously load the list (and theoretically cache it). I even tried downloading the list of values hoping it would use that - but I just can't get it to show lookup values in the subform when offline.

Anybody encounter this? I really don't want to have to hardcode the options in a dropdown as the list is fairly dynamic.