Im not an expert, but DVD rips with makemkv should be mpeg2 no? . the synology 918+ has enough horsepower to transcode a single 4k stream in software without using the native hardware transcoding of the celeron j3455 processor. I can play all of these mkv rips with the ds video app, just not via dlna.
but the codec theory does sound plausible though I just haven't gotten to the bottom of it. maybe there is a synology side setting that I have yet to find