My guess it is due to the low temperature. With this being a floor, the actual temperature of the wood and polyurethene is probably even colder, especially if the space below is unheated.
I would try to raise the temperature in the room plus use fans for extra ventilation to try to get it to dry.
Also, test the remaining poly by applying some on another piece of wood and put the wood in a warm location (70 degreees) just to see what happens.
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