There's two related (possibly the same?) issues here. Deselecting the data track option shifts the data tracks to the main tracklist when it shouldn't, and it's not possible to add/remove data tracks.

To reproduce:

• Open a release with a disc ID, go to the tracklist tab
• Select "This disc contains data tracks at the end"

Expected result: You can add more data tracks, but not normal tracks.
Actual result: You can only have the one data track that was automatically added.

• Deselect "This disc contains data tracks at the end"

Expected result: The data track is removed, because the disc ID limits the number of tracks.
Actual result: An extra track has appeared which can't be removed, so the release now has more tracks than the disc ID.