Problem: Replacing album artwork in Picard's right pane (matched release) consistently fails to embed the image, despite Picard displaying it as New Embedded and showing green checkmarks after saving.

Steps to reproduce

1. Load and match 13 FLAC tracks to a release.

1. Select all tracks in the right pane.

1. Replace their artwork with a local 1500 × 1500 JPEG (Replace enabled).

1. Save all tracks.

Result: All 13 files save without embedded artwork, verified independently using Python/Mutagen. Saving an individual track again without reassigning artwork also fails.

Working alternative

Move all 13 tracks to the left pane, select them, assign the same JPEG, and save.

Result: 13/13 files successfully receive embedded artwork.

This behavior was reproduced repeatedly, including with freshly re-encoded FLAC files containing no previous artwork or macOS custom-icon metadata.

Relevant preferences: Embed images into tags and Embed only a single front image are enabled; artwork replacement restrictions are disabled.

A custom post-save plugin copies existing native FLAC artwork into ID3 APIC frames. It does not generate native FLAC artwork.

Additional observation: After a failed right-pane save, Picard still displays the image as New Embedded. Moving the track to the left pane causes its Artwork tab to disappear until the image is reassigned.

This suggests that right-pane artwork changes may not be propagating to individual files during saving.

Is this a known Picard 3.x issue or regression?