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.
Been a while since experimenting with this!
We work around this by having our Vector Tile Package sideloaded to mobile devices and the webmap set to use the local vtpk when going offline in FIeld Maps Designer.
To be honest, I'd forgotten that I couldn't get this working (and just thought it did) but doing some quick tests now before replying shows that when trying to take a map offline with our custom basemap (ESRI Imagery + custom VTPK in the maps basemap group), it errors out on Field Maps. When changed to just the default Imagery basemap, it worked fine. So apparently no, it still doesn't work.