From MBS-2216

Steps to reproduce:

Go to an existing release
"Add disc" to create second disk
Click "existing tracklist"
Navigate to album with disc title (e.g. The Fragile (CD2: Right) by Nine Inch Nails)
Click "add disc"
Go to advanced editor
>> The disc subtitle is not copied from the source release

Possibly it should be? This is somewhat debatable, since the subtitle is an attribute of the release/medium, not the raw tracklist, which is what you're really adding. I'm not sure whether it'll be more annoying trying to clear this subtitle in the cases where it doesn't apply, or more useful to have it copied automatically.