I have the same problem with a Mybook NAS and Twonky upnp. All files are playing except Wav. I tested Wav files from various sources but every time the same behaviour: 2-3 seconds play without sound and then it stops. Are you sure this is a tagging problem? I know people where any Wav file plays without problems.
My hardware is based on RPI 3 and Hifiberry Digi+ pro as well. Is it possible this is hardware related?