Currently, offline basemaps only work for maps displayed in the Collect area. In the Inbox, Outbox, Draft and Sent areas, the map viewer only work with online maps. This is an old Survey123 problem, which didn't receive the proper attention.
As Adilson notes, the Inbox currently does support basemaps outside of Survey123's defaults. We will be looking at addressing this (along with other issues with the Inbox's maps) in a future release.
