Setting an instance of an asset in a level to "Is Editor Only" will cause the asset to not be present in the level in a packaged game, but the asset is still included in the cooked content for the game.
The setup in the reproduction project attached is a single map with a static mesh that is set to Is Editor Only. The map is included in the "List of maps to include in a packaged build" array and "Exclude Editor content when cooking" is enabled.
This also occurred in 4.18
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-57041 in the post.