This is a problem I have with %movementname%, %movement%, and %showmovement%. Everything works correctly for %work%.
I have, for example, an album that I have recorded, with three multiple-movement classical works on it, rendered as ALAC files for this example. I tag it with mp3tag, including the new iTunes tags. When I add these files to iTunes, a few files show up correctly but most are missing the movement name and number and the showmovement tag does not take effect on these files.
I manually edit the missing information in iTunes, and mp3tag shows the original information. dbPoweramp's ID-Tag then shows two tags for each of these elements, one supplied by mp3tag and one supplied by iTunes (I do not have a lower-level mapping of these tags than the ID-Tag descriptions): ID-Tag reports the original "showmovement" with the iTunes "Show Work Movement", the original "movementname" with the iTunes "Movement Name", the original "movement" with the iTunes "Movement Number", and iTunes also adds "Movement Count" (which I did not originally supply) with a value of 0. mp3tag only sees the originals it put there in the first place, and I confirmed by changing them in fresh copies before importing to iTunes, tagging in iTunes as before, and the result was two distinct versions of each tag.
dbPoweramp appears to read the atoms created by iTunes correctly but these appear to be different than what mp3tag writes.
I tried reading a hex dump of one of the ALAC files and it appears that Movement Name is mvn, Movement Count is mvc, and Show Work Movement is shwm, but I have not (yet) found the iTunes value for Movement Number. The mp3tag %work% maps to ©wrk according to main_tags.html and appears to be accepted as is by iTunes.
UPDATE: I have attached a binary file (renamed as .txt so it would upload, but please open with a hex editor) with extracts from the metadata encountered in one of the files. The tags added by iTunes are at the end (after a 16-byte "line" as displayed in a hex editor as all 00, and they are ©mvn, ©mvi, ©mvc, and shwm. All original tags are in the first portion of the file, including correctly written ©wrt etc. but the problem tags seem to have munged keys "M----", "I----" and "s----" with further information marked "mean" then "com.apple.iTunes" then "name" then, respectively, "showmovement", movement", and "movementname". I do not know if these were created by mp3tag or interpreted from the original tags and inserted by iTunes.