I did the following:

• open a release group (in this case, 2f2907a8-acc6-454d-85ab-53fc6f771f02)
• add a release
• use the existing release's track list as a template
• add two tracks
• move the tracks to their correct position, using the green arrow buttons
• save the result (note that the step where I associate recordings has the new tracks at the end, mis-numbered)

If you now look at http://musicbrainz.org/release/75741535-c84e-414d-9ee9-ff2adfd3618f, you see that the tracks are not sorted by track number.

Worse, this affects export to Picard: export is ordered as shown, not by track ID. The result looks like in the attached image and causes recordings to be misfiled!