Although no one has jumped in to comment on this, just to close the loop I wanted to say that since the release of 25.1.9 this issue seems to have resolved and the routing is behaving as you would expect. My guess would be that one of the two changes below are responsible for the corrected behavior.
- system: fix "weight" default fallback causing non-string return in gateway status
- interfaces: refactor newwanip IPv4/v6 scripts to reduce differences between them