Ironically this is exactly how I am too. Even at work, if I'm talking through a problem on a presentation or with my boss, I'm much more scatterbrained, and I'll try to dodge those kinds of calls with "Just give me 30 minutes and I'll figure it out." which always goes better for me.

That said, now we're just talking about take home challenges for interviews and you always hear complaints about those too. And shorter, async timed challenges (something like "Here's a few hours to solve this problem, I'll check back in later") are now going to be way more difficult to judge since AI is now ubiquitous.

So I really don't think there's any perfect methodology out there right now. The best I can come up with is to get the candidate in front of you and talk through problems with them. The best barometer I found so far was to set up a small collection of files making up a tiny app and then have candidates debug it with me.