It takes time to a) initialise and b) get an idea of the total of files to be scanned, etc. before it can possibly place any % scale.

So that takes time and you will see a delay before it is displayed, personally I never bother worrying about this function. Though I have never seen it remain at 0% until completion.

How do you know it doesn’t scan it ?
Presumably you aren’t sitting watching the whole scan.

Since you don’t give us any idea what your Custom scan settings are, so we can’t really be sure of anything.