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You sealed out the foragers who were coming back into the hive. Robbing is a very aggressive activity, you will see bees tussling, literally. There will be bits of wax deposits on the floor of the hive being robbed out as wax cappings are opened.
Not really sure what is going on with your hive, would need to see pictures.
They may have swarmed, hence the lack of numbers as you say, but you may have a replacement queen as evidenced by them bringing pollen back in. Bees bringing pollen back to the hive is usually a sign you have a laying queen or one about to but you need to do a thorough inspection and look for the queen, or eggs and open brood to tell you, you have one for sure.
Bees that are queenless will go into survival mode and will bring in pollen and even make honey so you will have to check to see if they are queenright.
...JP