So, fully deleting the navmesh bounds, and re-creating it with the same parameters, solved this problem.
I have no idea why, and that’s quite frustrating. Trying to debug this without stepping through source is also quite opaque, as the output is just … nothing happens.
that p saved my ■■■ 
Yep, for some reason this worked for me too. No other changes were needed.
[Encountered the bug when doing “Introduction to AI with Blueprints” by Chris Wilson - Navigating the Mesh module]
Thank you, this solved the issue.