Apparently, this is no feature of ProTools, this actually is within the AAX specs.
In the AAX specs of UAD, I read following: “Multi-mono support, so you can use UAD plug-ins on surround and stereo tracks with full linking and unlinking capability.”
That is exactly the same behaviour as to be found in a VST3 plugin.
And … Nuendo add the extra functionality of the Routing editor, where you can chose how many, and which, channels to be used.
The question that should be asked is why Third Party Manufacturer xxx doesn’t provide true VST3 support.
Fredo