Troubles with tftp server in latest FreePBX distro

I’ve updated 3 FreePBX servers to FreePBX 13.0.167. Somewhere along the path, The tftp servers have stopped working, but with some strangeness.
Two of the servers are connected via a IAX trunk (all on an internal LAN) - let’s call them A and B. The third is on the same LAN but standalone - call it C.
A and B can tftp to each other. C can’t TFTP to A or B, and vice versa. All can TFTP to themselves.
No other devices on the LAN (windows or linux) can TFTP to any of A, B or C - including all of my phones!
This is true whether the firewalls on ABC are enabled or disabled (service iptables stop).
SELINUX is dsabled.
I’ve watched the log, and the TFTP server is never started on failure, even the the client claims it is connected (verbose mode.) The client eventually times out.
Any idea what’s going on or how to debug this?

Never mind. I found a “smart” switch that needed to be reset. I guess it wasn’t passing UDP?

On a related note - is it understood why the Windows (10 only?) tftp client doesn’t work right? I get timeouts for 4 of 5 windows machines I tried. The request goes through, but data times out, and for 10 byte files.