The support for the Linux subsystem with the Windows Firewall API is quite new actually. Perhaps you need to run a Windows update and reboot if it’s a fresh yet outdated Windows install? I’m guessing that could be the issue since nobody else has reported this.
Or, maybe after running the updates try a clean install of GlassWire after you reset the Windows Firewall, and then choose “reset firewall” with our installer also.
Meanwhile we will continue to try to recreate this issue on our own machines. Thanks for your report and sorry for the problem.