I think I have a work around for the issue
You might be able to get around the issue.
Garmin Express builds the map img file on the PC before it sends it to the Edge.
On my Windows machine the file that it builds is located here
C:\ProgramData\Garmin\CoreService\Downloads\Map\Map_Map.CYCLEMAPNA.2018.10\RMU\CYCLEMAPNA\2018_10\bc4c6426-7f3a-4721-be23-5b142e4ab0be\IMG\006-D4677-11
The file is called GMAPPROM.IMG.
After the update had failed using Garmin Express I manually coped this file to the Garmin folder on the Edge 820 and it worked.