EDIT: Please ignore this.
With mip-mapping enabled, some GPUs (such as my Intel integrated graphics) cause a visual bug with indexed textures that use alpha. This bug has been present for a while but not noticed before. All indexed alpha textures in MTGame will now be converted to RGBA.
Issue https://github.com/minetest/minetest_game/issues/1397
We advise that in your mods you use RGBA mode for any texture that contains transparent or semi-tranparent pixels. You may wish to convert your existing textures.