Two points:
- unrestricted fasttracking is incompatible with use of mangle rules because most packets belonging to fasttracked connections skip a big deal of firewall processing. Few don’t, which means that the connections via the non-default routes work but with many dropped (because misrouted) packets
- you connection-mark the packets in chain=forward but routing-mark them in chain=prerouting; as prerouting comes first, the initial packet of each connection, which causes the connection-mark to be assigned in forward, is not routing-marked afterwards, so it takes the default route. Subsequent packets of the same connection are routing-marked because the connection they belong to has been previously marked, so they take the marked route (which may be the same like the routing-mark-less one or different)
You may want to read this post regarding selective fasttracking