I am reviewing your post and I noticed you mentioned this aspect:
If ARC is not working even on local WiFi without Tailscale being involved at all, then it sounds like there is a deeper issue than just Tailscale, and something is preventing communication between the Server and the ARC device.
Since you have a Linux Server machine, I would check to see how the behavior is like if you try to temporarily host the Roon Server on another machine (Windows/Mac), or double check the firewall configuration on the PC. I know you mentioned it was off, but it sounds like something is still blocking the connection. If you confirm the issue only occurs on the Linux Core, then the following articles might help: