This issue was discovered by users of Serum 2, which bundles the Instrument and Effect versions of the plug-in in a single binary. Confirmed by Serum 2 dev team with WaveLab Pro 12.
The Serum 2 FX plug-in does not appear in WaveLab’s plug-in list.
This appears to happen because Serum 2’s IPluginFactory lists the Instrument classes first. As a workaround, if we modify Serum 2 to return the Effect classes first, it then appears in WaveLab.
This is clearly a bug in WaveLab so felt it worth reporting here. It seems like WaveLab stops scanning a plug-in bundle as soon as it sees an instrument and doesn’t check to see what else is contained.