There is a beracha that many Ashkenazim (especially in Chutz Laaretz) say as the final beracha of Birchot Keriyat Shema of Maariv (before the Amidah) while many Sefardim do not say it.
Should a (e.g.) Sefardi say Amen to this beracha i.e.הַמֶּֽלֶךְ הַמֶּֽלֶךְ בִּכְבוֹדוֹ תָּמִיד יִמְלוֹךְ עָלֵֽינוּ, when in a shul where they says it, when he hears it from the Chazan?
For example, in Yalkut Yosef Kitzur Shulchan Aruch 25:54, and OC 422:4, he writes that Sefardim shouldn't answer amen to Ashkenazi Birchat Hallel when it is half-Hallel, and shouldn't answer amen to Ashkenazim making a beracha on their Tefillin Shel Rosh (because many Sefardim do not say these berachot themselves).
The principles behind not saying amen to a beracha you do not say yourself - do they apply here?