Part of the fbx textures become gray after the rig in mixamo

When I click the back button once, to go back to the placement of the rig points, it’s then become gray, but it reverts back if I redo everything again. I’m not sure if this problem has anything to do with my fbx file from blender or not. I hulled the character in magicavoxel in order to save loading times when implementing the character in a game, but I feel like it shouldn’t impact the surface, especially since the lower body is hulled too but there’s only the issue of the textures mid-way-leg upwards. Is there any solution here, maybe about how to export the fbx file, what to include and not? For context I’ve clicked off animation, I have -z as the forward as otherwise the animation became really messed up, and I’ve only selected mesh. The rest I haven’t touched, but some aren’t included by default for me so it might be that I have to click on some of them?