OS?

Mac OS Sierra 10.12.1

Versions.

angular-cli: 1.0.0-beta.20-4
node: 6.9.1
os: darwin x64

Repro steps.

Using SCSS:
Have a component's scss file with a prefix-able property (for instance: display: flex);

• Running ng serve, the output will have various prefixed CSS properties for display: flex;
• Running ng serve --prod, the output will only have the actual written display: flex, without any prefixes.