Well, I'm able to reproduce this problem at will now. Unless someone can explain to me what property I'm not using correctly that is leading to this behaviour, I'm left with no other conclusion to draw other than there is currently a bug with both the Modern and Classic flexible height gallery controls.
Here is the test:
Insert a flexible height gallery. Connect it to the data source of your choice.
Insert just four controls:
A 2px rectangle at X=0, Width = Parent.Width
A label with the data of your choice from the datasource. Y=2. Width = Parent.Width. Height = 17 (you could likely change the height without effect.
A label with data of your choice from the datasource. Y=Label1.Y + Label1.Height. Width = Parent.Width. Height = 29(you could likely change the height without effect.
A button with data of your choice from the datasource. Y=Label2.Y + Label2.Height. Width = Parent.Width. Height = 30.
This will display correctly in the Editor when you press the Alt key and it will display correctly if you click the Preview app button.
Well, well. Just before I clicked Post, I decided to try one more test.
If I do the test detailed above in a brand new empty app, the gallery behaves correctly. But when I do this in a pre-existing app, no matter which flex height gallery I choose, the test fails.
So, I now know there is some old reference in this app causing the gallery to not render correctly. The questions are:
1. Can I find it at all and modify it so the app behaves?
2. If I can't find it, will it somehow follow along if I try and recreate the entire app in a new empty app by starting with a blank canvas and copying and pasting controls?