Some say that Mar'it Ho'ayyin can be applied by Posqim while others restrict this [somehow] to only what Hazal specified in the Talmud.
Although this debate has been attributed to a S'phardic - Ashkenazic dichotomy, this dichotomy may be misleading
See SA YD 87:4 re: a Mari't Ayyin issue in conjunction with Mother's Milk
See B'er Hagolah who quotes source as Shu"t Rashba [see more below]
See Be'ur Hagra 87:9 who says see SA YD 66:10
Also see GRA there 66:14
See Beth Yosef YD 87 D"H Katav HaRashba citing Shu"t Rashba 3:257
Quoting Rashba as saying "Mistavra"
BY [apparently] endorses this with "Zeh Nir'eh Li Barur"
See Darchei Moshe 2
Who questions this
1 Rashba promotes this al pi svara
2 BY endorses it
3 GRA seems to attribute it to a Talmudic Analogy