This bug happens if you try to create a new release by duplicating an existing release that has tracks without duration.

Reproduction steps:

• Go to the Recordings tab and select a new recording for the track
• Go to the Tracklist tab, add a track duration for the same track
• Go to the Recordings tab again, the track will be attached to the previous recording instead of creating a new recording like you selected in first point

It doesn't necessarily have to be the initial recording. If you change the initial recording into some other recording prior to this whole process, that recording will be attached again instead of creating a new recording.