This is something that was already an issue with ArcMap, but is now also present in ArcGIS Pro: the dialog for "Label Priority Ranking" cannot be resized and is generally to small.
Also, I have never understood what the order of the presented classes was based on the first time you open the dialog. I would expect it to be sorted by order of the layers in the TOC, as the drawing order of layers is also generally speaking an indication of the "priority" of its associated labels. Nonetheless, the layers appear in a kind of completely random order?
It would also be welcome if it was possible to drag & drop multiple(!) label classes at the same time to another position in the vertical list.