By default, FlowPane resizes all children in each row to fill the height of that row. A rowValignment of BASELINE is specified to override this behavior:

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public final void setRowValignment​(VPos value)

The vertical alignment of nodes within each row of a horizontal flowpane. If this property is set to VPos.BASELINE, then the flowpane will always resize children to their preferred heights, rather than expanding heights to fill the row height. The property is ignored for vertical flowpanes.
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This is not working. A FlowPane continues to resize its children to the row height even when rowValignment is BASELINE.

This was first reported as a doc bug here:

https://github.com/javafxports/openjdk-jfx/issues/348

After evaluating it, it is clearly a functional bug.