I have this same issue, and unfortunately, the solution would be to delete the Motor6Ds and rewrite them with the limbs in the correct position. From what i’ve seen, this bug happens only when you insert character rigs while having Team Create Enabled, and trying to copy and paste a working model doesn’t work either. Since you have tried importing it directly with roblox instead of a plugin, this is clearly an issue on roblox’ end.
However, applying an animation on the broken rig seems to fix it (an animation made on a seperate fully functioning rig). The rig’s static state seems broken forever though. Hope this helped.