Why enemy can place minefields on top of my minefields? Is it a bug or its intended by game mechanic?
you can stack minefields of different players, this is intended, but you can prevent other from laying minefields, if you have an active minesweeper. Minesweeper can freely move thru minefields, but you only allowed to have 1 minesweeper active, Other minesweepers will die to minefield if you try to move thru, which is a bit annoying.
It was one of the last changes, before this, you could have used more minesweeper active running thru minefield, but it was nerfed to improve minefields. For the same reason minefields get buffed from 500 to 1000 damage.
Btw its a new kind of gameplay in the fast world, you rush to minetech and lay minefields iaround the homeworld to prevent him from doing anything. Also if the attacker sends a single minesweeper, the defender is not able to lay its own minefield.