Have you ever had an idea you started to explore and it was only halfway through that you found you couldn't quite make it work so you just tie up loose ends and go with what you already have?

Usually I know what the ending of a story is. If I understand your question correctly, I wouldn't usually start writing a story while it's still just an 'idea.' Part of being a professional is that you don't go sailing with just the compass, it's kind of expected that you also build a boat.

That said, I've had stories where the central conceit didn't come across as clearly as I'd hoped, and occasionally a story will be hijacked for editorial reasons, although I do tend to plant my feet to avoid that last one.

The "tying up loose ends," thing, that does happen sometimes in serial fiction, it can't really be avoided. It sucks when it's a bad situation like the second volume of Birds of Prey, but when I'm given enough time and space, it can be meaningful, like the first volume of bop or the end of the Secret Six series.