I tried to reproduce this, but couldn't unfortunately. This error happened when I deleted firewall rules regarding established and related traffic to replace then with the state-policy settings.

I started using VyOS because I saw Ansible gained support for configuring VyOS devices. I want all my devices in config management. I'd lie if I said I was familiar with the inner workings of the Python module that interacts with VyOS, but from my config management POV I think it makes sense to login into a normal shell and then perform 'some action'. This action could be done in a CLI tool or it could be something OS-related. So for me, I think it makes sense to login to a normal shell first. My two (possibly worthless) cents ;)