Update: Added an additional case where the same question could be asked.
It says in Shulchan Aruch 208:12 that when one makes an after-bracha after eating food for which the fore-bracha was "mezonos" or after drinking wine (for example) and it is Shabbos, Rosh Chodesh, or Yom Tov, then these days are mentioned in the after-bracha. The Mishna Brura writes there in note 58 that if one didn't mention these days in the after-bracha then they are yotzee b'dieved. The Shaar HaTziun there, in note 60, points out that this is from the Magen Avraham and that the reason is that one wasn't required to eat this particular food on that day. He cites another reason: that there are those that hold anyway that one doesn't have to mention these days in the after-bracha.
My question is as follows: After drinking the last 2 cups of wine at the Pesach Seder, an after-bracha is made. If one didn't mention Pesach in the after-bracha, would they need to say it again? The reason I might think so is that one is in fact required to drink these two cups, thus obviating the first reason of the Shar HaTzion to not require repetition of the bracha. However according to the second reason I suppose one wouldn't have to make the bracha over again. Are there any poskim who discuss this? Would, according to the first reason that the Shar HaTzion brings down, a person have to make the bracha again?
Similarly the question could be asked in regards to the bracha made after eating Mezonos by a Kiddush on Shabbos. Since this is technically considered to be his "seudah" (in that he made Kiddush and now needs to have a seudah in order for the kiddush to be good.) that is he had a chiyuv to eat. If he left out "Shabbos" from the bracha that he makes afterward would he have to say the bracha over again? Seemingly it's dependent on the 2 reasons the Shar HaTzion brings and perhaps would say since many hold you don't need to mention the day at all then over here in this case we would be rely on that opinion to not say the bracha over again. Are there poskim that discussed this case?