SCSS Opacity build bug?

Opacity is not run by percentage but by decimals so:

• opacity: 1; is 100%
• opacity: .5; is 50%;
• opacity: 0; is 0%;

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.