Why most cursive forms of two Chinese characters, "不" and "正", have a different shape that violates the stroke order?
There might be a bit of misunderstanding about where cursive script comes from. Many cursive characters do not actually come from regular script, but are derived from Warring States era brush calligraphy and seal script shapes.

This here

cannot approximate the shape or follow the stroke order of
秦簡23.1睡虎地秦簡東漢隸嵩山太室闕　
as the left part isn't as emphasised as this:


This here

cannot approximate the shape or follow the stroke order of
篆正部說文解字西漢隸孫臏37
銀雀山漢簡
nearly as well as this: