After several attempts and a good night sleep, I had the idea to just lower the DPI to 300... and the imagery basemap showed up in the exported map! When I did it again at 600 dpi, no basemap.
Just out of curiosity, why is that?
Is the Imagery Basemap the one from ArcGIS Online (i.e. World Imagery, etc.)?
FAQ: Is it possible to improve the resolution of a basemap from ArcGIS Online?
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ArcGIS Online basemap source
Yes, the Imagery Basemap from AGOL. I didn't realize that the service layers were optimized at 96 dpi. "Issues may arise when attempting to print or export maps with service layers, as the map printed is in low quality. Most cached web map services such as those available from ArcGIS Online are designed for viewing on the screen at this resolution and are expected to print at a lower quality because the services are not designed for printing at higher quality. The basemaps are provided as services, thus such behavior is expected."
Thank you for answering my curiosity George Thompson!
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