“Question – According to the Piskei Teshuvos [that I may continue eating without a new bracha if I had such intention while making the bracha achrona], why when I bentch al hakois do I make a borei pri hagafen on the wine [if I already drank during the seudah]? Surely I plan on drinking it, and the birchas hamazon will not stop that?”
Maybe it’s because you specifically had in mind that the original bracha should not cover the cos shel bracha. You can ask the same question by the arba kosos by the Seder. Perhaps it’s the same answer there.