IMG_Load_RW can't load images not divisible by 8, but IMG_Load can

[FrostKiwi]

I'm having a strange problem with my emscripten, C program, which loads images to display via WebGL 1.0. The images I have in the virtual filesystem of emscripten and load via IMG_Load here. Works fine, even for NPOT textures and dimensions not divisible by 8. I even made sure WebGL doesn't complain with glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

However, users are supposed to upload their own images, which I receive in a memory buffer and load via IMG_Load_RW here, which receives a buffer of the file uploaded here. The images loaded with that function however, have their memory alignment mangled. I get skewed images, ones with RGB boundaries mixed up ending in a rainbow mess, etc.
So I'm confused what I'm doing wrong, if IMG_Load works fine and loads everything perfectly, why does does loading from memory via IMG_Load_RW fail?