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After keeping polyhives 30 years, I have seen that often.
Polyhive "water balance" is different because wooden hive soaks water droplets inside the wood.. Box is wet, but you cannot see it. In polybox water must be leaded out down the walls and then out.
There are several ways to get water inside
1) slit, where rain can enter
2) condensation inside, and often cold air moving via slit makes condensation water.
If that happens, I mostly open that "water cup" that water can drill down to the floor.
Condensation happens in cold corners and it is better to break those "cups" which gather water. Those gather mold during winter.
3) Too few bees in big space and condensation happens too easily inside the hive.
4) too much ventilation, which keeps interrion cold. Dew point is then inside the hive.
Restrict the room so that bees fill all space and keep them warm.
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5) If you look after rain to the solid bottom, you may meet water pools on floor. Rain has entered via slit between box and bloor.
6) I can seen often too, that the floor board comes out and makes a landing board. It gathers rain water and it may drill inwards.