In all Slot's talk about the low block, he's never addressed why we concede goals against them.
Scoring against a low block is hard, granted, but not conceding against a low block is literally the easiest situation you can manage from a defensive point of view. He never talks about that though, just says things like "one chance and they score, we should do better...".
Good teams suffocate the opposition if they choose a low block, particularly against the sides at the bottom of the table. They should never be allowed to get out of their own half.