Managed switch works for Linux but not for u-boot?

I had the weirdest of a problem. Two computers communicating with each other over ping and TFTP works. When I boot one of them into U-boot (a bootloader that supports TFTP boot) it can’t ping not load tftp of the other machine complaining on ARP timeouts.

I swapped with a dumb switch - all works. Everything else (machines, cables) are the same. The managed switch is a Cisco switch and I have a serial console to it, but I’m not familiar with managing those switches - what feature is potentially blocking u-boot's arp packets?

I’ve double checked with tcpdump - the other machine never seer u-boot's arp packets, but does when the same board is booted into Linux. I’ve also checked Cisco's monitor event-trace arp continuous and it didn’t print any packets but it did say link status went from up to down to back up when I rebooted.

Is there some sort of Mac filter on Cisco switches?