Connect problems occur when a particular friend enters your Minecraft server.
Connect problems occur when a particular friend enters your Minecraft server.
You're experiencing unusual latency when certain players join your game. Despite stable connections for others, specific individuals cause ping spikes to 120-140 ms, disrupting gameplay. Once they disconnect, ping returns to a normal 21 ms. You're on fiber with AT&T, but still see speeds around 947 Mbps down and 945 Mbps up. The issue seems tied to network routing or device settings. Since you can play without problems otherwise, consider checking your ISP settings, router configuration, or even testing with a different connection. If the problem persists, look into potential routing issues or firewall rules affecting your traffic.
I've noticed Windows tracert often gives misleading results in this situation. Your observation matches what I was thinking as you described. It seems like your ISP might be creating a loop-back, causing your traffic to get mixed up. I don't have a concrete fix, but it's possible you're dealing with an ISP routing, MPLS, or OSPF problem. I'm not sure how much your provider can help right now. The easiest option could be to stay connected through a VPN when accessing the server.