Minecraft: Mac -> Windows won't connect; Windows -> Mac will. Why? [duplicate]

Question

Any time I try to log in to an open lan world I am now unable to. It was very hit and miss in 1.16 and all the 1.17 snapshots simply do not allow multiplayer between Windows and Mac.

I can see the lan world which is running on the windows 10 machine from the mac launcher.

but trying to connect to it times out

no amount of rebooting works. The mac can PING the windows machine happily.

The launch arguments are

-Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M -Djava.net.preferIPv4Stack=true

the windows 10 machine is stock standard. no third party firewalls or antivirus has been installed. No special lan settings have been changed other than when installing the wifi dongle on it I selected that it was connected to a home network. That's it.

If I HOST the game on the mac, the Windows machine will NEVER see it on the lan via the launcher. I have to edit in and put in the IP address and PORT.

The WINDOWS machine is able to join a game hosted on the MAC. The MAC machine is unable to join a game hosted on WINDOWS.

What else can I try, since I want the WINDOWS machine to be the server?

/edit

For those who asked, here are the specs and more details.

Mac Specs: MBP 2015 Running Mojave, 16GB ram, AMD Radeon R9 M370X 2 GB. PC Specs: AMD FX 4200, AMD Radeon 580 8Gb, 16GB ram.

I have checked:

• that the IP address of the windows machine is correct. I used ipconfig at the command prompt to get the IP address and pinged it from the mac. This is fine.
• I got a windows laptop (windows 7) and put the Minecraft client on it, logged on with my account, and it was able to connect to the PC server no worries and without bringing up any firewall messages or anything.
• The problem occurs connect mac->widows.
• The problem does not occur windows->mac or windows->windows
• The error code is not mentioned on the Is there a list of error codes for Minecraft? thread
• Other types of connection from the mac to windows work just fine on the same lan.