I can solidly recreate the blank preferences symptom on my rig.
Currently (rev 5.4-323-g43bd7f5) built from git.
If I have more MIDI Ports than can fit on the prefs dialog the symptom will always appear. When this occurs, the project overview at the bottom of the editor also disappears as if my screen has infinite height. That is, only once I’ve attempted to display preferences. Kinda ruins things. I can always get it back by toggling prefs attached/detached without the device with a lot of PORTS connected.
I hope this info helps.