I have also noticed this problem, e.g. a slight click or pop between tracks.
I have just burned an audio cd from 320 mp3's using MM 18.104.22.1686 and the "pop" has got worse and is much more noticeable
Burned the same tracks using another burn engine (Roxio) and no "pops" so it is MM which is "popping"
I also tested by converting a couple of tracks to wav and no clicks or artefacts found
(Using MM to convert & Audacity to test)
I do have "On the fly burning" enabled and "level volume" (89db) selected.
I am using "Disk at once" option but I have noticed the same problem previously using "track at once"
I hope this can get sorted out as its a real P.I.A. to keep swapping burn engines/programs