Atleast we have our nav savior

It's due to the slope of the stairs. There's a convar that controls the generation on a slope level. If you set the convar value to a lower number, the nav generate can complete staircases, but may generate some unusual areas on other slopes, as a side effect.

Change the convar of nav_slope_limit from 0.7 to something like 0.65, or even 0.5. And it'll fully navmesh stairs.

nice

some people can be sometimes good with maps but they are horrible with nodes & nav.

this is why its hard to find an actual good map to play.

Going by 3kliksphilip's video on the subject there are probably a LOT of maps with bad or outright broken navmeshs.