@denis1928: I have built a mental idea of why this could be happening.
* Stockfish calculates all the possible moves with "infinite" depth (infinite from our point of view of beginners).
* The moves are sorted, and one of them is chosen according to some "weight function" which depends on the difficult level.
* In lower levels there is a higher probability of chosing the 2nd-best move, the 3rd-best move or even another move down in the sorted list.
When we are in the middle of a piece-trade, the best reply for the computer is normally to "re-take"; even the 2nd best move is very often a material loss for the computer. And that is exactly how the lower levels behave, not only on this site
lichess.org but also on other chess sites: we take the queen and cross fingers, maybe the computer does not retake and we are 1 queen up :)
I wonder if there are more realistic ways of weakening a good engine like Stockfish.
Perhaps calculating the replies with lower depth, maybe depth=2, 3 or 5 instead of depth=22?