To use % heights in CSS, the parent element must have an explicitly defined height, or a % height that can be traced back through parents to a defined height, or something else specifying the size, such as a sibling.
In this case the size of the parent is auto, and depend on the size of it's only children. This is an undefined situation that the standard is trying to address.
Since the of the parent is unknown when calculating the flex-basis of the children, the algorithm ignores it. The mathematically correct solution would be having element of 0 or infinite size in most of theses cases, so most browser simply ignored the value. The min/max constraint are still applied afterward with the result that the text overflow.
The builder did seems to suggest the uxml has a definite size but when looking at the debugger the underlying asset is still Auto/Auto. This will be brought to the UI debugger team to see if there is some improvement that could be done on this regard.