I don't know at what point this issue started exhibiting itself, but for the longest time I thought it was some combination of Little Snitch rules and the iCloud Private Relay. But I uninstalled Little Snitch and disabled the Private Relay feature, and this issue didn't go away. So I reinstalled Little Snitch and told it to block 172.0.0.0/8 addresses, and that resolved my issue.
You see, I created a different thread about this a month ago, but my Windows 10 server has several virtual network adapters, and Synergy recognizes all of them, for some reason (but no IPv6...?). And while my mac