In short, what I want to know is can custom basemaps be enabled for offline use? I've followed the guide in this blog post to create and share Custom Basemaps with my org, but I don't seem to be able to use them in a webmap and have that map enabled for offline use in Field Maps.
Extra info: my custom basemaps either use the ESRI Terrain or ESRI Imagery basemaps with a Custom Vector Tile Package overlaid within the basemap group of the webmap share to the Custom Basemap Gallery group. The Vector Tile Package is enabled for offline use, as are the webmaps.