Not possible to add multiple images to a flac file

The manual page for metaflac implies that more than one image can be attached to any given flac file as does RFC9639. However this fails due to error opening the flac file.

The following example shows that I can add album art but adding artist art fails:

metaflac --import-picture-from="3|image/jpeg|cover|1411x1385x72/72|album_art.jpeg" 01\ Laundromat\ Blues.flac #works
metaflac --import-picture-from="8|image/jpeg|cover|450x550x150/150|artist_art.jpeg" 01\ Laundromat\ Blues.flac
01 Laundromat Blues.flac: ERROR: writing FLAC file, status = "FLAC__METADATA_CHAIN_STATUS_ERROR_OPENING_FILE"

The FLAC file could not be opened.  Most likely the file does not exist
or is not readable.

The following example, after removing album art, shows that I can add artist art but adding album art fails, demonstrating that both images are fine and correctly defined:

metaflac --remove --block-number=3 01\ Laundromat\ Blues.flac
metaflac --import-picture-from="8|image/jpeg|cover|450x550x150/150|artist_art.jpeg" 01\ Laundromat\ Blues.flac #works
metaflac --import-picture-from="3|image/jpeg|cover|1411x1385x72/72|album_art.jpeg" 01\ Laundromat\ Blues.flac
01 Laundromat Blues.flac: ERROR: writing FLAC file, status = "FLAC__METADATA_CHAIN_STATUS_ERROR_OPENING_FILE"

The FLAC file could not be opened.  Most likely the file does not exist
or is not readable.

The following example, after removing album art, shows a failing attempt to add both images in one command:

metaflac --remove --block-number=3 01\ Laundromat\ Blues.flac
metaflac --import-picture-from="8|image/jpeg|cover|450x550x150/150|artist_art.jpeg" --import-picture-from="3|image/jpeg|cover|1411x1385x72/72|album_art.jpeg" 01\ Laundromat\ Blues.flac
01 Laundromat Blues.flac: ERROR: writing FLAC file, status = "FLAC__METADATA_CHAIN_STATUS_ERROR_OPENING_FILE"

The FLAC file could not be opened.  Most likely the file does not exist
or is not readable.

It seems to me that the code assumes there can only ever be one image per flac file, unless I have misunderstood the manual page.

Start by reproducing the failure with the metaflac --import-picture-from commands shown in the issue, including adding the images in both orders and in one command. Trace the metadata-chain handling behind metaflac and verify that multiple picture blocks can be written without the FLAC__METADATA_CHAIN_STATUS_ERROR_OPENING_FILE error.