Just found this on Celemony’s website:
“Please note: with sample editors like Steinberg Wavelab, Sony Sound Forge, or Bias Peak the plug-in variants of Melodyne cannot be used. Please use the stand-alone variant instead.”
Fair enough. Apparently for Melodyne to work its magic, it needs tempo information from the DAW. Audio editors by definition don’t work with tempo in the same was a sequencer and therefore can’t be used to host the Melodyne plugin, so it’s not a bug, and that explains why it is listed as a “don’t load” plugin in WaveLab.