I'm also having this issue, but I'm using New Collector version 19.1.0 on Android. When I open Collector Classic on the same device, all of the side-loaded basemaps appear and can be used. In New Collector, the only option is "Default".
Hi Andrew -- Since you are using the Collector Beta, could you please post in the forum of the Collector Early Adopter community? That way the right people from the team see it, as well as other beta participants who might be experiencing similar things. Thanks!
(Collector Classic and new Collector don't share basemaps, so if you are expecting the same ones, that might be the problem)
Hi Kylie - My teammate posted an update there. Can we expect to use multiple side-loaded basemaps in the full version of Collector? Or will we only be able to use one tpk basemap?