Start is buggy for me both with and without Classic Shell it seems (I've tried multiple alternative Start buttons from CS to no success), so it must be Windows' fault at its core, I just thought that maybe there is some kind of a blind spot in the way touch active area is placed that can be eradicated with CS maybe, or maybe there is Windows bottom left hot corner in place here that is supposed to be turned off yet maybe still has some kind of a trace of itself left for some reason.
Here is a video I made to show the problem more clear (please turn on annotations):
https://youtu.be/SYpgVdPLMx8Start button size has been set to "555" with CS in order to make everything clear and visible even with my terrible camera. The problem is there regardless the button size, also it seems the blind spot on the bottom left corner scales with the size of the Start button, so that might be a hint to the root of the problem.