Steps to reproduce
This also happens if you choose a release with the same number of tracks.
The expected behaviour here is probably complicated. Since there is a TOC being attached, the track times etc in the "based on" release cannot be used. However if I'm adding a release with bonus tracks, I think I'd expect to be able to re-use the titles and have the editor discard the times. Being able to copy titles/artists is the safest way so that the tracklists can be shared using the "matching magic" in the database, I believe?
I think in this situation it should give you a warning.
I'd suggest something like the following on the release duplicates page; suitably highlighted:
"You are adding a DiscId with this release. If you base this addition on an existing release, the track times from the existing release will be ignored"
If existing release # tracks < discID # tracks => empty titles tracks should appear in tracklist editor
If existing release # tracks > discId # tracks => should pick the first "disc # tracks" and populate those in tracklist. Perhaps it should give you a warning that it has discarded some track titles.
But you should not lose the TOC/discId without any warning, which is what is currently happening.