Your evaluation peaks and then bleeds out — the advantage you earned never reaches the scoresheet.
You reached a winning position and let it slip. Converting a win is a separate skill from getting one. The method: trade pieces, not pawns, to simplify toward a clean ending; deny the opponent counterplay; and don’t grab extra material when a simpler path just ends it. When you’re ahead, simplify — when the win is in sight, take the safe road, not the flashy one.
The good news: it is a pattern, not bad luck — and patterns can be trained. Step one is measuring yours: how often it appears in your real games and how many rating points it drains. Then the daily repair: 5 minutes on your own positions, not invented puzzles.