Sounds like you are losing interest in the piece, and being away from it for a bit helps. You can also keep trying to find new thinks to listen for and think about in the piece, inner voices, different ways of phrasing, different tempi. Anything you can do to force yourself to pay attention to what you are doing rather than just running through it may help.
So I am curious, do you have a teacher or someone else to evaluate if your pieces are truly declining? How long does it take for you to sense a piece is worsening— just a few weeks like you said or actually longer? Are these pieces memorized? I have experienced what you describe, but only after playing memorized pieces for many months. In my case, only I have felt they have worsened. My teacher has not ever said to me I am repeatedly playing a piece worse than previously. Sorry for all the questions