I am testing for siphons I think you're right. I just don't get how it's happening now after running for 3 years fine.
Have you changed the height of the blue bucket? Was it sat at the same level as the sump before?
Was the fill tube always in the sump? Was it always at the same height in the sump?
Were the water levels in the ATO reservoir and the sump always the same?
Hopefully that is the problem as that’s an easy fix, and that’s the important thing.