Hello,
If I put a new mp3 file with ID3v2.4.0 tag to the playlist,
the CD number is not shown, if I reenter that field, it is shown.
Here are the first bytes from such a file:

0000:0000 49 44 33 04 00 00 00 00 0f 76 54 50 55 42 00 00 ID3......vTPUB..
0000:0010 00 13 00 00 03 43 61 66 65 20 44 65 6c 20 4d 61 .....Cafe Del Ma
0000:0020 72 20 4d 75 73 69 63 54 49 54 32 00 00 00 10 00 r MusicTIT2.....
0000:0030 00 03 45 6e 74 72 65 20 44 6f 73 20 41 67 75 61 ..Entre Dos Agua
0000:0040 73 54 52 43 4b 00 00 00 02 00 00 03 3154 50 45 sTRCK.......1TPE
0000:0050 31 00 00 00 0e 00 00 03 50 61 63 6f 20 44 65 20 1.......Paco De
0000:0060 4c 75 63 69 61 54 41 4c 42 00 00 00 1c 00 00 03 LuciaTALB.......
0000:0070 43 61 66 c3 a9 20 64 65 6c 20 4d 61 72 20 2d 20 Café del Mar -
0000:0080 54 68 65 20 42 65 73 74 20 4f 66 54 50 4f 53 00 The Best OfTPOS.
0000:0090 00 00 02 00 00 03 31 54 44 52 43 00 00 00 05 00 ......1TDRC.....
0000:00a0 00 03 32 30 30 33 00 00 00 00 00 00 00 00 00 00 ..2003..........
0000:00b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

Also the field in the dialog containing the CD number is not adjustible.

Forgot to tell the version I'm using
amarok-beta3c
taglib 1.4