Tags on flac not being indexed

[jurope]

I have around 1700 albums, mostly ripped CDs in flac format. I tag using id3v2 on debian. Around 200 are MP3 from podcasts and so.
All the MP3 are indexed correctly, but only a few of the flac make it. The rest appears in the [untagged].

Any suggestions?

MinimServer 2.0.18 on a trial license
running on
Synoilogy RS3617RPxs, 6.2.3

id3v2 of a track not indexed:

$ id3v2 -l 'Abba^Super Trouper/track02.flac'
id3v1 tag info for track02.flac:
Title : The Winner Takes It All Artist: Abba
Album : Super Trouper Year: 1980, Genre: Unknown (255)
Comment: Track: 2
id3v2 tag info for track02.flac:
TPE1 (Lead performer(s)/Soloist(s)): Abba
TALB (Album/Movie/Show title): Super Trouper
TYER (Year): 1980
TIT2 (Title/songname/content description): The Winner Takes It All
TRCK (Track number/Position in set): 2/13


looking at the mcache (cutting from od -A x -t x1z)

109770 72 20 54 72 6f 75 70 65 72 2f 74 72 61 63 6b 30 >r Trouper/track0<
109780 38 2e 66 6c 61 63 00 00 01 77 e5 9d 8d 53 00 00 >8.flac...w...S..<
109790 00 00 01 8b 28 80 00 00 ac 44 00 00 00 02 00 00 >....(....D......<
1097a0 00 10 00 15 88 80 00 00 00 00 00 8b 56 78 00 00 >............Vx..<
1097b0 ac 44 00 04 66 6c 61 63 00 00 00 00 00 00 00 00 >.D..flac........<
1097c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
1097d0 00 00 00 00 00 00 00 00 00 03 00 00 00 71 00 1f >.............q..<
1097e0 41 62 62 61 5e 53 75 70 65 72 20 54 72 6f 75 70 >Abba^Super Troup<
1097f0 65 72 2f 74 72 61 63 6b 30 32 2e 66 6c 61 63 00 >er/track02.flac.<
109800 00 01 77 e5 9d 8d 38 00 00 00 00 02 26 78 80 00 >..w...8.....&x..<
109810 00 ac 44 00 00 00 02 00 00 00 10 00 15 88 80 00 >..D.............<
109820 00 00 00 00 c6 4d 88 00 00 ac 44 00 04 66 6c 61 >.....M....D..fla<
109830 63 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >c...............<
109840 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
109850 00 00 03 00 00 00 71 00 1f 41 62 62 61 5e 53 75 >......q..Abba^Su<
109860 70 65 72 20 54 72 6f 75 70 65 72 2f 74 72 61 63 >per Trouper/trac<

and of the first part of the flac (ID3v2):

000000 49 44 33 03 00 00 00 00 02 2a 54 50 45 31 00 00 >ID3......*TPE1..<
000010 00 05 00 00 00 41 62 62 61 54 41 4c 42 00 00 00 >.....AbbaTALB...<
000020 0e 00 00 00 53 75 70 65 72 20 54 72 6f 75 70 65 >....Super Troupe<
000030 72 54 59 45 52 00 00 00 05 00 00 00 31 39 38 30 >rTYER.......1980<
000040 54 49 54 32 00 00 00 18 00 00 00 54 68 65 20 57 >TIT2.......The W<
000050 69 6e 6e 65 72 20 54 61 6b 65 73 20 49 74 20 41 >inner Takes It A<
000060 6c 6c 54 52 43 4b 00 00 00 05 00 00 00 32 2f 31 >llTRCK.......2/1<
000070 33 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >3...............<
000080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
*
000130 00 00 00 00 66 4c 61 43 00 00 00 22 12 00 12 00 >....fLaC..."....<