my CRS368
I see no such product on MikroTik's site, even if I search the "Archived" products.
I thought that by putting both devices on a VLAN that only they use, all communication would be isolated.
I think I need a network diagram to make sense of what you're putting down in prose. Label the subnets and associated VLANs, and mark where the DHCP servers and clients are.
My best guess from your wall-of-text is that you've got more than one DHCP server on the same VLAN, and the client isn't talking to the one you think it ought to.