After examining a number of files it appears to that the "Picture Bits Per Pixel" tag is causing errors with reading the embedded artwork in the file. MC is writing to that tag. MP3Tag strips it out and also strips out the picture dimension tags. and the DBpoweramp converter writes the dimension tags but not the "Picture Bits Per Pixel" tag. Is it possible the writing this information into the file is new behavior from MC?
Here is the EXIF info:
File 1 (embedded art not showing in poweramp):
Picture Type : Front Cover
Picture MIME Type : image/png
Picture Description :
Picture Width : 700
Picture Height : 700
Picture Bits Per Pixel : 32
Picture Indexed Colors : 0
Picture Length : 905796
Picture : (Binary data 905796 bytes, use -b option to extract)
File 2 (working, tags resaved by mp3tag)
Picture Type : Front Cover
Picture MIME Type : image/png
Picture Description :
Picture Width : 0
Picture Height : 0
Picture Bits Per Pixel : 0
Picture Indexed Colors : 0
Picture Length : 905796
Picture : (Binary data 905796 bytes, use -b option to extract)
File 3 (working, encoded with dbpoweramp)
Picture Type : Front Cover
Picture MIME Type : image/png
Picture Description : Cover artwork
Picture Width : 700
Picture Height : 700
Picture Bits Per Pixel : 0
Picture Indexed Colors : 0
Picture Length : 905796
Picture : (Binary data 905796 bytes, use -b option to extract)