Or basically, just try reimporting the character again but this time you can try using another humanoid character, dont use the one from roblox default since that one seems to have some wonky things going on?
Moon-animator seems to work fine in importing though and the charcter inserter plugin.
Well this is weird can you send the rig file and texture via FileBin I am blender veteran I can help.
Most prolly you’re exporting the textures from a R 15, exporting it via R 6 will work but this can be possible that the file has messed up texture management or something. I can tell you more after I have the file open in my blender.
After uploading the files send me the link via private messages or here