I’m aware that there is an issue with tilemaps and transparency but this new behaviour I have spotted is really weird. I’m sharing it here in case either there is a known fix/workaround or if this will help investigate the behaviour.
In the first screenshot the tilemap is behaving as expected and is covering fully the stars in the background layer.
In this second screenshot you can now see the stars through the tilemap objects. The only difference is that I have moved my character a bit to the left.
If I shuffle left and right around this point I can switch between the two states consistently. There is another point on the map further left where the switch goes the opposite way around (left = opaque, right = translucent). Beyond these points the tiles behave as expected.