I know it's an old topic but I had the same problem. What I did to bypass this, was setting the opacity of the tilemap in an event.
(condition)On start of Layout -> (action)TilemapName - set opacity to 20
So when the game starts construct will set the opacity of the tilemap to 20
If you have like more copies of this on tilemap in your editor but you only want one particular tilemap with 20 opacity you can pick that one particular by its unique ID. Like this:
(condition) On start of Layout - (condition)TilemapName - Pick instance with UID 320 -> (action)TilemapName - set opacity to 20
The tilemaps UID is displayed in the Object type properties windows usually on the upper left side. I used 320 as an example.