We had this problem on a SFO-HAM trip late last year. Due to one 45 minute LH segment, our outbound 6 32kg allowance for two passengers was 2 23kg bags on the return. Despite the receipt also saying that status would get us 3 bags each, both UA and LH said there was absolutely nothing we could do about it.

Oh, I should note that we didn't actually get charged because I was able to keep it within the smaller limit. But it took some effort, and I didn't get to buy some things I was planning to because it was not worth the extra fee.