I'm not able to use "Highlight" shortcut. No matter I press control+command+h or control+command+u, it results in underlining text. Clicking the "Highlight" button also underlines text. I can highlight text using highlight with specific though.
Try clicking the highlight button (while not having any text highlighted) and select the highlight colour. It'll show a check mark beside the one you choose. Then unclick it and try the highlight shortcut again and it should make the default that colour highlight.
I just find you can select no text then change default highlight type. So it's not a bug and problem solved.