Do not use that feature. The point of using Tor is that you do not trust your VPN provider. Currently Tor only supports the TCP protocol. UDP (used in WebRTC for voice and video sharing, the new HTTP3/QUIC protocol, etc.), ICMP and other packets will be dropped. To compensate for this, VPN providers typically will route all non-TCP packets through their VPN server (your first hop). This is the case with ProtonVPN. Additionally, when using this Tor over VPN setup, you do not have control over other important Tor features such as Isolated Destination Address (using a different Tor circuit for every domain you visit).
The feature should be viewed as a convenient way to access the Tor Network, not to stay anonymous. For proper anonymity, use the Tor Browser, TorSocks, or a Tor gateway.
Source: How Do VPNs Protect Your Privacy? Our VPN Overview - Privacy Guides