Update:
Ok, I figured it out. It turns out that I got confused by GIMP's layer mode. The picture that I claimed "when I place them one on top of the other in any image editor, it looks perfect" is using "default" layer mode. If I change to "legacy", it's the same with what it looks like when in game.
I don't have too much understanding of the two modes GIMP offers, but note to myself that I should probably always use "legacy" when work on maps in the future.
I feel bad if someone actually spent time trying to look into this... Sorry about that!
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Original post:
Hi all, glad I've found this great plugin for my mapping, thanks for creating it!
However I just have one problem that I cannot seem to find a solution to: When I add ground and parallax layers, the semi-transparent shadow in the parallax layer doesn't get displayed correctly.
To reproduce this issue, use "demo_ground" as the ground layer, and "demo_par" as the parallax layer.
demo_ground:

demo_par (it's the shadow in this image doesn't work right with the plugin):

When I place them one on top of the other in any image editor, it looks perfect:

However when I put them in a map with the plugin and run the test:

Somehow the shadow of the tree is off. It should be pretty clear that the color of the shadow changes abruptly on the tile edge. It seems like the shadow in the parallax layer is more opaque than it should be.
All I did was I put the two images in their organized folders and added "<par>\n<ground>" to the map note. That's it, I'm not using any plugin commands. (Should I?)
I have been searching for a solution or an alternative for a while and have failed. I spent months on the map and really sad that I can't get it to work... Could you help me out please? Thanks for your time!