Issue with GIMP and Transparency

[ODISeth]

I'm doing my first sprite rip with Slay the Spire, and I was able to successfully extract all the assets from the game files pretty easily. However, one thing I'm beginning to notice is that attempting to pull some sprites into GIMP to assemble into a sprite sheet adds additional transparency that was not in the original sprite. Here are a few examples.
 
Here is the sprite for the "Blood for Blood" card. On the left is the raw file extracted directly from the game, and is how it's supposed to look. That is how the sprite appears when viewing it with Microsoft's built in photo viewer. On the right, however, is the sprite after importing it to GIMP. Note that I did not make any manual edits, that's just how the card appears in GIMP itself. All of the 000000 perfect black pixels in the middle has been replaced with complete transparency, and some pixels on the outside have also been replaced with partial transparency with a clear pattern, but not one I can understand. It appears to occur with more than just black though. Here's another example.

Again, left is original sprite, right is after importing to GIMP. This time the replaced pixels don't seem to be just black ones. The gray shadow was removed, but so too were some huge chunks of blue within the arm. I've never seen this issue before when working with GIMP for other projects, and I don't presently have access to any other photo editors to assemble spritesheets in. I don't want to just upload each sprite individually, but this issue is happening with a TON of sprites and I have zero clue why. I've tried changing GIMP's import settings and color management settings, I've messed with each sprite's layer attributes and overall image, nothing fixes it. If I had to guess I'd say there's some sort of metadata tag in each sprite that forces GIMP to index the images in a way that removes certain colors, somewhat like a heavily compressed GIF, but that's just a guess cause I don't see any indications of that in the files themselves.

If anyone knows what the issue is, please let me know. I'd love to preserve this game's assets but this is really making it difficult. I've attached the original sprite files for the two examples above if anyone wants to take a look at the metadata, see if there's anything there I missed.