@viragomann AHA! I figured it out now! So, that client (10.247.1.13) used to have my wireguard server running on it, and I never uninstalled it. So I THINK that ubuntu server had static routes set up for traffic on the 10.66.66.1/24 subnet, and was sending traffic to those subnets into the void. After uninstalling wireguard on the server, pings are now working between my windows machine connected via wireguard and the server at 10.247.1.13. Still can't ping windows to windows, but I'm guessing that's a firewall issue and I can look at that in my own time.
Thanks for the help folks! I think we can consider this resolved now.