With the 9.1.016 update from 2019-10-25 (and maybe earlier versions since I don't know when this started but it used to work), the WPSymbolDlgEx dialog does not display properly until you resize the dialog. Only the first row appears, but then the whole grid shows up once you start resizing the dialog.
FWIW, the dialog could use a slight tweak... the encapsulating border and Subset dropdown right side is right on the right edge, and the buttons are right on the bottom edge. Personal preference is to have some and equal spacing around the whole dialog. ;)
Also, everything is "relatively" huge (perhaps that is due to my use of a 4K monitor at 100% scale and you are drawing the dialog based on screen size which would be good...so just checking if that was intended).
One other question... is there a way to have the dialog centered based on the main form or better yet, the owner form? On a 4K monitor it throws the dialog way up to the left corner.