I play on PC with a controller and after a lot of searching, I found out that the button to reset the affixes, in the Masterworking option, is only available when I use the mouse. When I move again with the controller, the button disappears again. This happens not only to me, but also to the two other persons I play with that also use controller.
Don’t know which controlled you use, but I plugged my PS5 one and there is a button for masterworking:
https://imgur.com/a/yq7xZC3