We only have one but there everything is fine but it could be that the rubber is not in properly or there is water outside the rubber that is still dripping out.

My experience is that the cup is not always leak-proof when carbonated drinks are in the cup. With still drinks, the cup was leak-proof for me.

No, we have never had this experience. We had 4 such cups and all of them were always leak-proof. After 2 years, we gave them to friends and never heard that they leaked. We always cleaned them by hand, never in the dishwasher. Could this be your problem?