ah OK, thanks — I’m surprised I didn’t know about that issue before now.
And yeah about the parser difference, I had read the note at https://wicg.github.io/urlpattern/#ref-for-concept-basic-url-parser in the spec:

The URLPattern constructor string algorithm is very similar to the basic URL parser algorithm, but some differences prevent us from using that algorithm directly.

But then it seems there must be a bug in your pattern.test() implementation, right?
I mean for the following:

const pattern = new URLPattern({ hostname: '{*.}?example.com' });
console.log(pattern.test('custom-protocol://example.com/other/path?q=1'));
// false