`jpeg` generates 12-bit JPEG with 0xFF AC Huffman symbol about libjpeg

[thorfdbg]

Yes, your observation is correct. What you see here are the "default Huffman tables" that include codes for symbols that are unused in the baseline and extended Huffman case, but that are actually used in some XT extended processes like the "high bitrate case" for residual coding of large bpp cases. This way the encoder can get away with a single default table instead of multiple tables.
At this version, you can avoid the trouble (if it is one) by creating an optimized Huffman table (with the -h option) and only those symbols are included in the table that are actually used.
As you state, the standard is not very expicit whether that is permissible or not. It would be interesting to see whether that causes breakage in some implementations. Do you have further information?

[thorfdbg]

Won't fix. Work around documented if needed, though the streams should be fine.