I am on RUTX_R_00.07.19.2 with a RUTX50. Failover works when I disable my Wireguard VPN connection, VPN connection works when I disable all failover entries.
When both are enabled, the routing or firewall-rules prevent any clients connected to the RUTX to reach the internet (the RUTX can).
There are many similar threads and claims have been made that it was solved in earlier firmware versions. I believe it still stands and is easily reproducible.