This is indeed a good technical way (method of elimination) to debug things however caution here. Using a user → VPN will burn the VPN. It then should no longer be used for user → Tor → VPN. Burn here means the VPN is already tied to the user’s ISP assigned IP address.
Do we cover this in documentation somewhere? “Don’t reuse a VPN that has been used for user → Tor → VPN again for user → VPN → Tor.” Or more generally for a tunnel link?