I use opacity: 100% or opacity: 40% in some SCSS code in my React app.
Works find in development mode.
But when I build, all the elements have their opacity set to 1%!!!
I've worked around this easily by using rgba colors and setting opacity there.
But, what is up with this???
Opacity is not run by percentage but by decimals so:
See more on opacity css spec here: https://developer.mozilla.org/en-US/docs/Web/CSS/opacity
Sass never changes property values based on property names (see this example). I'm guessing you're using cssnano as well, in which case you're running into this bug for which a fix will be released soon.
