I'm thinking something like this: Evaluation of the position is even. All moves except one lead to high centipawn loss. Some of the moves look solid when evaluated superficially (too simplistic evaluation heuristics / not thinking enough ply ahead).

Lichess, chesstempo etc don't do a lot on the "superficial evaluation" - that's generally a hard problem to solve algorithmically.

Instead the main approach is to just mix new problems into the pool and let users try them. If there are no plausible alternatives, everyone gets the right move and the problem quickly becomes too low-rated to be shown to most players. If there are tempting alternatives and the correct solution is hard to calculate, it will become high-rated and only shown to stronger users.