I took a quick look and everything seems to be working fine in v6.10.2 (see screenshots).
Are you certain you aren't experiencing interference from other styles declared in your theme?
Mendix also sometimes creates surrounding divs, which means that the element in the modeler doesn't always correspond 1-1 to the actual html element. That can also cause some unexpected behavior if you are used to other platforms.
If you're still having trouble figuring it out on your own, feel free to share your specific scenario. Maybe someone here can spot the weak point.
Element in modeler with section of property bar.
Element in client with styles as shown in dev tools.