Get rid of nav mesh tiles

I work on reasonable small space, but nav mesh showed a tiled green space available for my AI.
Does anybody help me to fix this issue?
I looked at settings, I don’t really know what to do there. I tried testing different numbers, blindly, without knowing what I am looking for. Help!

I played with “settings” - cell size - it doesn’t solve my issue. The green space split into tiles in the first floor and I see no visible reason.