I can confirm, loading a save prior to jumping into the pool of water then traveling backwards and changing party members resolved it for me as well. Thank you!
Created an account just to post the same. Defeated Kostya and open the sluice gates but the quest doesn't update and if I leave for the garden Solace keep is destroyed. I also tried a few prior saves and ran into the same bug.
Basically at a standstill in play.
