.relatedTo() not returning tags with spaces
In your working example, you’re mapping request.tags to the search param, but in the non-working example, you’re mapping it to the slug param.

Those two params are handled in very different ways. search will do some normalization routines on the value, and perform a very loose search for those keywords (see https://craftcms.com/docs/searching for more details). But the slug param will actually look for an exact match.

So if request.tags is set to "low confidence" and you pass that value into a slug param, I have complete confidence that it will return zero results, since "low confidence" isn’t actually a valid slug (see what I did there?).

You would have slightly better luck passing request.tags through the kebab filter, which will make the value look a little more like a valid slug. But the search param is probably a better bet unless you know for sure that request.tags is actually going to be a slug.