I'm using ArcGIS Pro 2.5.1 The single layout in the aprx has four map frames, three small ones nested inside a large, whole-page map frame. When I export to PDF (or to EPS, then Distiller to PDF) the result shows the basemap in the small map frames, but the large map frame does not have the basemap. The basemap image that I am using in all of the map frames is Bing Maps Road. In ArcGIS Pro it looks fine - all appropriate layers on checked On for visible. However, when I check the layers (using Acrobat XI) the basemap layer for that data frame did not get written to the PDF, though the basemap is of course present and visible in the other data frames.
With more experimentation I see that it is explicitly about size. If I lower the paper size of the layout from 158" to 140" then the basemap shows up. However, last week I successfully produced some PDFs for maps that were 175" that had more map frames and similar amount of vector data.