Well I couldn't find this on the internet anywhere-

I was having issues setting up active-backup bonding on my Mikrotik router via this guide.

Whenever I turned on both interfaces, I experienced packet loss.

The solution was to TURN OFF hardware offloading. It doesn't work with active-backup. The docs even say it doesn't work with hardware offloading, but it failed in a way that was hard for me to debug.

So yeah! Now works great, router <-> unmanaged switch with bond bridged.