As long as the blend mode is not translucent or additive, it simply won’t give you the result you want. Normally, opacity mask is for either 0 or 100% visible values and opacity supports anything in between. However, according to the documentation in subsurface based models, opacity will instead be used do determine how much light can scatter through the surface. While the documentation on shading models is outdated, considering how you said that opacity appears when selecting the cloth model, I’m going to assume that cloth is also a subsurface like shading model.
in other words, as long as you don’t select translucent as your blend mode, the function of opacity will be different and won’t give you what you want.