If you add something like

let test = 'hello';
console.log( test );

to Special:MyPage/common.js you'll get

JavaScript parse error: Parse error: Missing ; before statement in file '<file>' on line 1

This is expected per T75714 and not what this bug is about. However, if you preview your /common.js with this code, it actually does work. This suggests that previewed JavaScript does not go through ResourceLoader's minifier.