I just upgraded Enterprise from 10.7 to 10.7.1 on Saturday August 17, 2019. Our custom basemap gallery has living atlas content (Human Geography) coming from a send-recieve collaboration group pair between portal and agol. The custom basemap gallery does appear in the Portal webmap.
When signed into either org or portal, the custom basemaps appear in the basemap gallery widget in my 2.11 and 2.12 wabs. If not signed in, the basemap gallery in the wabs bounces to the default esri basemaps. A similar question is posted here:
In the Web App builder for ArcGIS space but is a bit dated, so I thought I would ask here in the Enterprise space. This was not an issue for us from 10.6, 10.6.1 and at 10.7.