Problems are often "sticky" (A term I've read referencing this exact thing, a problem where you need to begin implementing before you understand it fully, but I don't think it's a widely used term) but I think the implementation step is a different matter than architecture stuff regardless. It's not that there aren't important things to work out, but for me I broadly consider that to be part of the implementation.

I get what you're saying though, I definitely don't think I'm solving all my problems before ever starting the code, but I definitely always finish solving the problem before I finish the code, and usually early enough that the rest of implementation drags even if there are quirks and edge cases to work out.