I don't use stock but I also use fedora, I had issues with the kill switch. There's two different kill switch options, my setup works much better with the simpler of the two. Also, I want to say fedora comes with the firewall enabled, plays into it being well ranked from a privacy standpoint I believe
That could be your issue, go to: settings -> connections -> select the VPN your connecting to -> VPN (openvpn) tab -> advanced -> general tab -> check use TCP connection
If the issue continues, definitely check the other advanced settings for sure. If you still have an issue I'd hit up Proton using https://protonvpn.com/support-form
Edit: Shield options are directly next to the kill switch icon under connect/disconnect as well