mwan3: cannot force output interface for targets in connected networks

Maintainer: @feckert @aaronjg
Environment: 21.02.1 x86_64, mwan 2.10.13-1

Description:
Even specifying a rule to exclusively direct a route through an interface, the router is still using other interface when the target is in its same network.

 ┌─────────────┐       ┌─────────────┐
 │             │       │             │
 │   R1        │       │    R2       │
 │             │       │             │
 └───────────┬─┘       └──┬──────────┘
             │            │
             │            │
           ┌─┴────────────┴┐
           │ eth0      eth1│
           │               │
           │               │
           └───────┬───────┘
                   │
           ┌───────┴───────┐
           │               │
           │      C1       │
           │               │
           └───────────────┘

In my case, C1 has a rule to always use eth1. When I ping R1 from C1, the router is still sending traffic through eth0 and not eth1.
It looks like mwan3 has a mwan3_connected ipset that includes all locally connected networks and skips rules in that case.
This is what it is doing. I wonder if this is what mwan3 should be doing.

Start by reproducing the C1-to-R1 case from the issue and tracing how mwan3 handles the mwan3_connected ipset for targets in locally connected networks. Compare that behavior with a rule intended to force eth1, then establish whether honoring the rule or documenting the current behavior is the expected outcome.