Sometimes, the paragraph beginning יִשְׂמְחוּ בְמַלְכוּתָךְ ends with זֵכֶר לְמַעֲשֵׂה בְרֵאשִׁית, and sometimes it doesn't. The only exception I could find composing and researching this question was Shacharit for Sefaradim, where it isn't mentioned. There may be other variations.
- Nusach Sefard here has it in Aravit, Shacharit and Mussaf
- Nusach Ashkenaz here has it in Shacharit and Mussaf (not, יִשְׂמְחוּ בְמַלְכוּתָךְ is not present in Aravit at all)
- Nusach Edot Mizrach here has it in Aravit and Mussaf.
Why isn't it in every Amidah (apart from Mincha), and why is it in the ones it is in and not the others? Don't forget to also explain why it is sometimes said at all - what is its purpose in being in our Amidah in יִשְׂמְחוּ בְמַלְכוּתָךְ in the first place?