So I figured it out yesterday but I wasn't allowed to post until today 😑
My hunch was correct and NetworkManager was doing something to the interface. I'm not sure exactly what, but once I excluded the interface from NM, all was fine.
Since I have direct control over the root fs, all I had to do was open up /etc/networkmanager/networkmanger.conf and add a line like this. ofc sub in the correct interface mac address
[keyfile]
unmanaged-devices=mac:f2:b7:0b:2f:07:d2
Also, while I'm typing this out. I initially used the sigmaris uboot for spi, but it didn't work well at all. the ethernet was constantly timing out and I couldn't successfully pull the kernel images via tftp. but the ayufan uboot worked much better.
