I'm using UI for WPF 2018.3.1016.
In the Crystal theme, the RadPasswordBox.IsTabStop property defaults to False. This is different than in the other themes I've tried (Material, Expression_Dark), where it defaults to True.
In the Material theme, the RadPasswordBox's watermark foreground is different than the color of the RadWatermarkTextBox's. It's obvious when you have both controls visible in the same UI, such as in a login form.