When Level::PointInSector method is called with actor's origin as parameter, while actor is standing very close to 2nd sized line (on the border of another sector), sometimes it returns a pointer to that, neighbor sector, not the one actor logically is in. I am saying "logically" because sometimes actor technically cannot pass even a little in the neighbor sector, when it has much higher floor, for example. In such case that actor's Sector pointer too is a pointer to that neighbour sector. Can this be fixed?

I think this has to do with actors having "square" boundaries, if even a little bit of the square is crossing the sector boundaries then the functions marks the actor a it's there, I'm not very sure if that's how it works though.

No, it uses it's origin for this.