In our game here (https://blight.ironhelmet.com/u#/game/5579523440508928), a Necromancer Lord took over a hydra pool at Secret Pool. We were quite worried our swamps would be flattened by six fast-swimming hydras, not that we had much left in the swamps by then, but instead they joined the same stack as the Necromancer Lord.
Since there were already trolls in the stack, it has then taken a gentle saunter down to the bottom of the swampland at 16 hours per square, while we all wondered what would happen. And what has happened? Well, the stack has happily proceeded onto dry land, hydras and all! The players in that corner are quite concerned that hydras will have the same combat effectiveness when being portaged as they do while swimming.
I’m guessing this is a bug, and that where it went wrong is when the hydras were originally allowed in to the same stack as the walking units.