Disc Subtitles displaying as tracks in Naim app on iPad but correct on iPhone

[Motel Blues]

I've successfully used Disc Subtitles in combination with Disc Numbers to differentiate different versions of an album within the same folder previously, but for some reason tonight it doesn't want to work for me.

I've got two versions of "Sgt. Pepper..." one with:

DISCNUMBER 1
DISCSUBTITLE Deluxe Anniversary Edition
TRACKNUMBERS 1-31

and another with

DISCNUMBER 2
DISCSUBTITLE 1967 Stereo Mix
TRACKNUMBERS 32-44

All the tag editing was done using MP3Tag, which reports the tag types as MP4.

When viewed with the Naim app on my iPhone, it displays exactly as I intended it to. However, when viewed in the Naim app on my iPad, under "Sgt. Pepper" I have...

01 >> Deluxe Anniversary Edition
02 - 32 all the tracks on the Deluxe Anniversary Edition
33 >> 1967 Stereo Mix
34 - 46 all the tracks on the 1967 Stereo Mix

Essentially, "something" is interpreting the DISCSUBTITLEs as track titles, and renumbering accordingly. I'd be grateful for any pointers as to how to resolve this. I appreciate it may well be a Naim app issue, even though other DISCSUBTITLEs are displaying correctly on my iPad.

Originally the two discs were in different folders; bringing them into the same folder didn't help.

I've restarted my iPad, the Naim app, the Naim streamer and Minimserver, and rescanned MinimServer. There's nothing related to the problem files in the MinimServer log. I've managed to achieve what I'm after previously (e.g. with UK and US versions of "Aftermath" displayed as separate discs on both iPad and iPhone), but I'm stumped here.

Some version numbers, in case they're relevant:
MinimServer: 0.8.5.2, update 123
Naim app: 5.10 (on both iPad and iPhone)
IOS: 11.4 (on both iPad and iPhone)

Thanks!