Previously Ogg Vorbis did not have any officially recommended method to embed cover art in the file tags. MJ14 still uses the old, more or less proprietary, method that was invented by JRiver many years ago. It creates a MIME encoded large text tag frame of the image file data, which is known to cause problems with some players. Starting from MC15 the cover art tag format was changed to the new official Vorbis format, which may work better, at least with newer players that are aware of this method.
I did a quick test and did not have any problems with removing the MIME encoded cover art tag by using the "Remove Cover Art" command in MJ14.
You did not explain how you sync the Android phone. If you use the Handheld feature in MJ14 and have file format conversion to Ogg Vorbis enabled, do you perhaps also have the "Cover Art > Add to Tags" Handheld conversion option enabled? This option will embed cover art even if the source files have only externally linked cover art.