kk, if here is no answers - then finger to sky )
if to add nm to sublevel - after generation it not saving, and if to reload ue - there will be no navmeshes
if to add nm to base level - it start to generate nm for sublevel A and saving em, but if to do same for sublevel B - nm in level A will be destroyed.
if to add one gigantic nm bound volume for all sublevels i have - it start to generate nm, even if sublevels not loaded (wtf?), so as result ue generating over 1.5 millions of some invisible navmeshes… kk, let’s try more…