We have quite a few basemaps in our Portal for ArcGIS and ArcGIS Online organizational account.
When accessing basemaps in Pro we see that the ESRI items display as expected, but most of the non-ESRI basemaps suffer from one of two issues, despite working fine in ArcMap and web maps.
A few display properly, a few clearly have projection issues (e.g. points across the US displaying in the Atlantic Ocean), and most do not display at all (white screen, may also be a projection issue). What I find strange is that they work fine in the AGOL and Portal platforms, through which they're being accessed.
Issue has been noted in Pro 1.0 and 1.1 beta.
My thought is that in some cases modifying the projection information in the basemap's JSON definition will fix the issue, but the one basemap that I attempted this fix with appeared unaffected.
This is an example of a service we have registered as a basemap that works in Portal and AGOL, but is shifted within Pro:
Most of the basemaps which don't display at all in Pro are MapBox-based.
Any thoughts are appreciated!