If they can't protect the 18 (including the retreat), in my mind, they didn't ever have 18.
The fall turn, in my mind, ends after builds and destroys, not after fall move.

I would agree that a win shouldn't happen until the phase change between retreats and builds of autumn. It should also be noted that some people wanting to draw should always wait until spring for the very same reason.

Yes, this is a bug in the website code. It has been discussed on the dev forum. It has happened to me on vdip in a 1 on 1 game. The dev forum discussion had a link to a game where it happened here I believe, although in that case I believe the person was very likely to win anyway, just not that turn, but my memory could be wrong.

The simple solution is to only check for a winner when the adjudicator is moving the phase to the build/disband phase or to spring moves if there was no build/disband phase. The downside of that is that most games which could safely be ended in the retreat phase are left going for that extra phase length.

A more complicated solution would be to go ahead and check after the fall moves, but a player can only be declared the winner if their number of centers minus the number of their centers that could be retreated into is 18+.

But for now we don't pay for the privilege to play here and I think we're getting a pretty good deal.

It's a bug since it's contrary to the rules, but, hey, just think of it as a WebDip house rule, like how talking is allowed during retreats and builds.

In the tens of thousands of games that have been played here and only a handful have had it happen where the winner was somehow illegit... Yeah, pretty minor bug overall.

"But for now we don't pay for the privilege to play here and I think we're getting a pretty good deal."
That's going into the profile...

And really in all of the cases where a winner is declared incorrectly only a fraction of those are going to have a situation where the person with 17 centers would not end up winning next turn anyways.