I don't have mp3tag. I'm using this program to deal with my tags, and I trust it:
I have this album in ALAC as my original source. I transcoded it to FLAC using Max. The tags are identical in each. Interestingly, the DX100 plays the ALAC tracks in correct order, but not the FLAC tracks. According to the Tag program, the tags are fine. I tried deleting all the tags, saving, then re-creating each tag manually. DX100 behaves the same.
Here's a screenshot of what Tag says (click to embiggen):