My goodness, that's sad and kinda defeats the purpose of sharing a holiday with people you care about in a meaningful way which I thought was the point. It's almost like they used the amount of food you ate as a barometer to evaluate the strength of the friendship instead of the using the proper measure of how you felt about them - the time you spent at their house on a holiday sacrificing some time with your immediate family.
I'd be thrilled if a friend popped in for a bit to have a drink, a few bites and a chat before leaving for their parents house for their main meal. I think that shows great regard. But only if I knew that beforehand when I extended the invitation, otherwise I could see my feelings being hurt if I put in a lot of effort and time on a dinner to find it not appreciated. The hosting couple if they didn't know beforehand might think the guest didn't like the food or company so was using "I have to go to my parents house and eat dinner" as a way to bail and head out for better pickings.
I agree and certainly would tell my hosts in advance, which I did, and since I wasn't the only friend invited, I didn't think my presence one way or the other made that big of a deal to my hosts, but apparenly so.
I felt particularly bad about one time because the friends were a gay couple, neither of which were in contact with their families, so in effect their friends
were their families, so I could see the impact might be more major to them. But then again, their situation didn't make my family go away or change my circumstances.
