The lesson
Focus Mode is deliberately unpleasant. It weights the puzzle stream toward whatever you are worst at, so your hit rate drops and stays down. That is not a bug: practice that feels easy is practice happening inside what you already know. The zone worth living in is roughly seventy to eighty per cent correct — high enough to keep going, low enough that something is being repaired.
Run it in short, dense blocks. Ten to fifteen minutes of weighted puzzles will do more than an hour of mixed practice, and past twenty minutes accuracy falls for reasons of attention rather than knowledge, which pollutes the weighting. Stop while you still care about the answers.
Use the playable reveal every single time you miss. When your attempts run out, the correct chord or scale lights up on the keyboard and stays playable — do not read it and move on. Play it three times, sing the top note, then play the thing you guessed and hear the gap. A miss you correct with your hands is worth ten misses you correct with your eyes.
Then narrow. If the stream shows you keep failing minor sevenths against a major backdrop, stop taking whatever comes and hand-weight the mode to that one item for a week. Broad practice maintains; narrow practice repairs. Alternate them — one narrow week, one broad week — and neither the weak spot nor the general fluency is left behind.