Just going to cross post this here for the sake of visibility!
Tl;dr setting “Navmesh Walking” mode doesn’t seem to work, and my characters will use “Walking” mode regardless.
It seems to be the same issue that this person had: https://answers.unrealengine.com/questions/533217/how-can-i-ensure-navmesh-walking-is-being-used.html but their fix didn’t work for me.