The key needed to be eased and you eased it. By pressing the damper pedal, the spring-loaded damper lever cannot assist in resetting the key. When I want to make sure that there are no sticking keys in an upright, I will hold down both the damper AND soft pedal. Doing this will reveal any keys that are sticking. The soft pedal holds the hammer butts up and keeps their weight from pushing the key back. In any well-designed piano, all the keys should pop up quickly with only the weight of the wippen on the tail end.
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