I'm sorry you're having trouble - especially when you're working on Christmas Day and there's no-one else around. I'm slightly surprised that there's no-one from Datawatch even checking the community. I'm afraid that I'm not going to be able to help you out directly either - I'm on holiday for the next few weeks and don't have access to a machine with Designer. There are people who can help - although time zones and holidays might make it a bit tricky. It might be a useful first step for you to let us know what version you are running (and any relevant details about the environment), and to prepare an anonymised data set that you can share so that others can replicate the problem.
The version is 18.104.22.16840
I cannot attach file because the Insert image action is disabled in this editor
Steps. to reproduce.
1. open designer
2. put a vertical stack
3. insert into the vertical stack a couple of horizontal ones
4. try to decrease a height of the lower one by mouse.
The height will be increased to Infinite value
I have found workaround.
But no worries, it is the smallest problem. There are lots of another bugs, issues, limitations and restrictions.