When exporting a Model with multiple textures to GLB with the option Single Texture File toggled on, the resulting texture image looks correct (the same than for OBJ with single texture file option), but the UV tiles all span the same 0-1 area. UV coordinates are not mapped to their respective tile for each texture (downscale each to 0.5 for a 2x2 texture, 0.333 for a 3x3 texture etc), therefore all faces from different tiles (different udim textures) appear on top of each other.
Probably similar to this 2 years old bug that might still have not been fixed yet GLB Export (textures misaligned) - #5 by EricHupe