If I call room_pack_load in the "base rooms" creation code and the base room doesn't contain the same number of tilemaps as the one being loaded none of the tilemap layers get displayed. Is there something I'm doing wrong like calling the function in the wrong place? Or is this behavior intended?

I found out why nothing is being displayed when there is no layers in the base room. The layers being created from room_pack_load have depths that are higher then the background layer. Because of the this the background is being drawn above it. I'm not sure why having 2 tilemaps already being created in the base room fixes this issue but maybe it pushes the background layer higher up? Still not sure on why I can't correctly read the layers though. My work around for that though is to store the tilemaps using room_pack_store_tilemaps and then reference those instead.

Hello, and yeah - whenever you add a new layer, GameMaker pushes the rest of the layers 100 depth units up. You can see the automatically set depth when you select a layer.

Manually adjusting the layer’s depth [to make sure that the background is, well, in background] is a good solution. That’s also done in layer properties.