Do the JPEG files open in Photoshop? If so, then the problem is most likely another bug in JPEG parsing in Canon's DPP code.
To get anyhwere with this issue you'll have to export the same shot in both LR5.5 and LR5.4 and then compare the xmp/iptc/exif/maker tags with exiftool - is there a difference? Of course Adobe will try to blame other apps, with "standards" such fuzzy as jpeg or tiff that's always possible until proven otherwise.
You can also simply strip all the tags from a "broken" LR5.5 image by running "exiftool -all= -Adobe:all= image.jpg" and then see if it opens in DPP or whatever.