Switched to an old Juniper SRX300, and the same configuration works fine. The dhcp relay packets now use the internal LAN side IP 192.168.5.1.
Is this a limitation with MikroTik?
I did some more research and found this post from 13 years ago:
http://forum.mikrotik.com/t/dhcp-relay/35564/1
I quote the last sentence from that post here:
If any of the RouterOS dev’s are wtaching, It would be nice if there was a tick box to allow this behavior
So is this forum watched by RouterOS dev’s?