Civ 6 Question about missionary and spreading religion. I used it on my main capital and it rose another religion instead

question on Civ 6 gameplay. i only have 60 hours of gametime and this is my first civ 6. spread across like 2 years. just fyi.

this is my first religion run. after having like 5 cities, i realized my main city was taken over by another religion. so i sent 2 of my missionarys there.

when i used their skill on both units, instead of spreading my own religion (taoism) it spread the one that was already dominate in.

what happened here? this is the first time i seen that happen, even if my religion wasnt dominate one. isnt the point of using your units to spread religion to take it over with your religion?