Initial sate: https://postimg.cc/Hrpxpjbs/b9ecca3a

Pay attention to  red node: Manipulating path above (below ) that node will affect the path below (above)  

State 2: https://postimg.cc/ctxxdb8k/33aa51d0

State 3: https://postimg.cc/1fYmZMRM/98460920

Why does changing the path below that point affected the path above that point?

Make the "red" node a corner node.

Thanks biy why was not it a corner node from beginnng?

All others were corner nodes

Because you're click+dragging in that case - even a tiny amount will turn it into a smooth node.

Can Inkscape create only Cusp nodes by default and not try to make them other types?

Here is a video of me trying to ranmdomly put nodes with Bézier curves and one of them is somehow was created as smooth node: https://disk.yandex.ru/i/2ACKm4qXnaMKjw

No Shift key or Drag was used 

That was a drag event... working more slowly will help.