Interesting. I will look into it. I am assuming that these are all 4K textures? DXT1 or 5 (and does this change the behavior)?
EDIT: Reproduced the issue, I think I've got it sorted. Next release will be fixed. Turns out that it is good to free malloc'ed heap memory copied into (and out of) emscripten compiled libraries (if only someone had told the dxt-js author ).