Gatozzzz,
Welcome to the forum!
The moving object needs time to be spotted by IVS, and tracked, BEFORE crossing the IVS line.
So IVS lines at the edge of the image can have problems. IVS may not have time to identify an object, then draw a box around the object, and then have time for the object to cross the line.
Additionally, the size of the box is a factor, since 50% or more of the box must cross the line before an IVS "cross line" event is triggered.
With that post in the middle of the FOV, and with such a large object box (tracking a human that is large wrt the FOV), you may not be giving IVS 1) enough time to identify and box the person, or 2) the box may not adequately cross the line.
I tweaked my IVS line placement while using a laptop connected WiFi to my home network. Then I surfed in to the NVR web interface gui. The laptop view showed the lines, and boxes, as I walked around with my laptop.
I observed the amount of time it took before I was "boxed" after I entered the cam FOV.
And where the 50% line cross criteria was achieved.
Generally, I'm pleased with IVS.
Note: my cams are mounted further away than yours. So a person, boxed by IVS, is 10% or 20% of the entire FOV image. And the box is entirely within the image frame. It seems your box, tracking a person coming up the circular stairs, would be a box that has its border at the very edge of the FOV image. That might be a challenge for the cam, and IVS processing.
Experiment, as described above. You'll figure it out!
Again, welcome!
Fastb