Multi-WAN - traceroute and ping from specific WAN IP / interface doesn't work?

[ZPrime]

The GUI's Diagnostics -> Traceroute option provides a drop-down to select the source IP for a traceroute. Same thing for the ping function.

It doesn't seem to work in a multi-WAN scenario.
I currently have pfSense's "Default Gateway" setting in System -> Routing set to a gateway group that is just basic failover, two tiers (one per ISP), only failing on a full outage.

However, traceroute via a specified source IP/interface doesn't even work if I change the Default Gateway to "Automatic."

It does work correctly out of whatever WAN is "chosen" as the DGW at the time, but I can't run a traceroute from the "secondary" gateway. Doesn't even seem to work from the CLI, I just get "* * *" responses for everything (I'm even using ICMP traceroute since I know some ISPs drop the UDP traces).

If I manually switch the gateway order / force the other WAN to be "active," that's the only way to get traceroute out of it.

Obviously I can ping anything "same-subnet" that BSD knows is local to each of the WAN interfaces, but it always wants to send all traffic out whatever default gateway is set, regardless of the specified source IP (or source interface, in the case of traceroute)...

Is this all expected behavior? If so, it's not really documented officially anywhere that I could find.