Not sure if this should be under MCL bugs, or under MC (Java Edition) bugs.  It affects the actual game, but the root cause seems to be in the Launcher.  It makes me unable to find other computers on LAN.

The issue is temporarily fixed by adding the Java Argument "-Djava.net.preferIPv4Stack=true" to the list of arguments.  After starting Minecraft back up with the arguments set, I can then connect to LAN immediately.

However, we shouldn't have to change Java Arguments to spin up a default multiplayer game with two fresh installs on a Mac and on a Windows10 PC.  Also, I've discovered that the Java Arguments reset every time I open the vanilla launcher, which is specified in a different bug (https://bugs.mojang.com/browse/MCL-13745).  That bug makes this bug more painful.

I am fairly certain that this is in fact the issue.  While my friend on PC had a running LAN game for several hours, every time I closed Minecraft, reopened the launcher, and didn't change Java Arguments, I couldn't find the LAN server.  Every time I closed Minecraft, reopened the launcher, and did add that to the Java Argument, I was able to connect to LAN immediately.  I did this about five times.

That Java Argument, "-Djava.net.preferIPv4Stack=true" should be included by default to make it consistent across platforms.