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These are the exact symptoms I had with a leaky Shimano calliper. See if it happens again with new pads.
The bad pads were in the front originally, I swapped them to the back.
If the calliper were leaking I'd expect the front to be crap again - but the problem stayed with the rear pads, regenerating over night.
Maybe the front pads were contaminated when the brakes were assembled? i.e. they got leaked on, but things are not leaking now, however the mineral oil is seeping back to the surface of the pad overnight having been burnt off during the day? IDK.
The rear pads have become shit again over night- interesting as they were spot on at the end of yesterday. Guessing the contamination has seeped up to the surface from the core of the pad. That or aliens.