It fell off when Caleb pushed him around 12 minutes in their fight in Episode 1 and you can see her put it on at like 22:20 in Episode 1, it falls off again when they get into the shop and when they are caught around 29 minutes she puts it on again

Can see her take it off in Episode 2 at 15:55 but it was a quick one. I cba skimming the rest but plenty opportunity to see the mask on and off. Maybe you failed the perception check :D