Hello frOsty. I have used this same feeder and have never had bees get into it. Like buzzbee said, make sure your cover fits tight on the box that is on the feeder. One question though. How did you get 2.5 gal. of feed in a plastic hive top feeder that holds 1.25 gallons of feed ?
I'm almost 100% sure bees aren't getting in from the top, it's completely sealed.
Also, I refilled 3 times, not all at once.