Playing Battle at the Dam with difficulty above Normal, let all characters start from Dam, but the screen with drag option to arrange the party sequence is deactivated without any reason I think. You can see it as usual but all characters icons are locked in position and touch input doesn't work.

Not yet verified at Legendary, but at Heroic for sure with my Android.

I think that is how they want it. Since one person need to stay at the dam, you need to leave one person behind to "watch the dam".

The screen I m talking about is before to play the scenario. After chars positioning in the locations, you have a screen with all characters icons in a row, there you can change the characters sequence through sliding them (first on the right, last on the left). In that scenario, I couldn't change the order and that doesn't change the scenario rules, just who is the first character you play at turn 30, then the second... and go on.

To change that you can still forfait, rearrange the party in party management and restart the scenario. Normally to avoid that you can set your characters orders of play after the location assignment just before to play turn 30.

Yup, that correct the party arrangement screen before the location assignment screen. It been like that since the major patch and I think it was how intend to lock people in the dam on higher challenge. It maybe a adverse result to that.  

Devs fixed on the fly in the new patch note! Wow thit is really awesome. Today noted, today solved