I’m having the same issue with all music only being scrobbled to now playing and not completing the scrobble to Listenbrainz from a Jellyfin provider. Issue persists for all music cached or not.
Logs for 2 different songs start to finish. Both songs appeared on the Listenbrainz website as ‘Listening now’ but once the next song starts, the listen wasn’t recorded:
debug-20230318_105109.zip (37.1 KB)
Your issue have nothing to do with that one, do not post on random closed issues 
Anyway your issue is on Jellyfin side:
2023-03-18 10:43:53.218 Verbose/JellyfinLogger: --> [480] POST https://jellyfin.tomobiki.moe/Users/da40ae2d01054a9c8a11e4ebc805348c/PlayedItems/6f69512f076df384f5f6b841f0717c83 (31-byte body)
2023-03-18 10:43:53.218 Verbose/JellyfinLogger: --> [480] {"DatePlayed":"20230318104353"}
2023-03-18 10:43:53.292 Verbose/JellyfinLogger: <-- [480] 200 https://jellyfin.tomobiki.moe/Users/da40ae2d01054a9c8a11e4ebc805348c/PlayedItems/6f69512f076df384f5f6b841f0717c83 (74ms, unknown-length body)
2023-03-18 10:43:53.293 Verbose/JellyfinLogger: <-- [480] {"PlaybackPositionTicks":0,"PlayCount":123,"IsFavorite":true,"LastPlayedDate":"2023-03-18T14:43:53.6513853Z","Played":true,"Key":"Various Artists-Urusei Yatsura Complete Music Box, Volume 1-0009-0243Melancholy no Kiseki"}
And
2023-03-18 10:39:53.819 Verbose/JellyfinLogger: --> [301] POST https://jellyfin.tomobiki.moe/Users/da40ae2d01054a9c8a11e4ebc805348c/PlayedItems/dd0047ff1f4d2a2bcca5d3af11eb24cc (31-byte body)
2023-03-18 10:39:53.820 Verbose/JellyfinLogger: --> [301] {"DatePlayed":"20230318103953"}
2023-03-18 10:39:53.889 Verbose/JellyfinLogger: <-- [301] 200 https://jellyfin.tomobiki.moe/Users/da40ae2d01054a9c8a11e4ebc805348c/PlayedItems/dd0047ff1f4d2a2bcca5d3af11eb24cc (71ms, unknown-length body)
2023-03-18 10:39:53.890 Verbose/JellyfinLogger: <-- [301] {"PlaybackPositionTicks":0,"PlayCount":342,"IsFavorite":true,"LastPlayedDate":"2023-03-18T14:39:54.2458545Z","Played":true,"Key":"\u6765\u514E-ALL OVER MELTY BLOOD ~ Melty Blood Actress Again for Limited Edition Original Sound Track.-0001-0038Strange Dreamer (\u5348\u524D\u96F6\u6642\u306E\u9022\u702C)"}
Symfonium send the played status.
Sorry about that, I was about to make a new topic when it suggested that one and I thought they were related. Thanks for the info, I’ll look into what’s going on the Jellyfin side. I thought it was Symfonium related because it seems to send scrobbles properly from my other Jellyfin players, but I’ll look into it more extensively.
It’s possible that Jellyfin have decided to diverge more on Emby and changed something without really seeing the impacts.
But I send the scrobbles manually to control the percent and to update them from offline playback.
After looking into this further, it appears that it still might be an issue on the Symfonium side? At least with the implementation of the Listenbrainz/Last.fm plugin for Jellyfin.
It seems the plugin looks for the PlaybackPositionTicks field to be not null in the payload from the player to Jellyfin (It also compares this against an internal 50% progress threshold for scrobbles). The Jellyfin Listenbrainz plugin logs that it’s receiving a null PlaybackPositionTicks from Symfonium when I complete a track and doesn’t submit the listen consequently:
[2023-03-18 18:24:35.264 -04:00] [DBG] [3] Jellyfin.Plugin.Listenbrainz.ServerEntryPoint: Playback ticks for '"Strange Dreamer (午前零時の逢瀬)"' is null
I think this is also confirmed from the logging snippets you sent earlier with the PlaybackPositionTicks field being 0 in both? I suppose the solution would be to just populate that field in the payload when reporting playback to the Jellyfin server from Symfonium. Any thoughts?
For reference when I play the same track through the Jellyfin webui I get this and a proper submission on Listenbrainz:
[2023-03-18 18:46:20.819 -04:00] [DBG] [26] Jellyfin.Plugin.Listenbrainz.ServerEntryPoint: Playback of '"Strange Dreamer (午前零時の逢瀬)"' stopped: played 100.00000318726991% (1411866669 ticks), required 50% or 2400000000 ticks for submission
[2023-03-18 18:46:21.527 -04:00] [INF] [15] Jellyfin.Plugin.Listenbrainz.ServerEntryPoint: User "Meloom" listened to '"Strange Dreamer (午前零時の逢瀬)"' from album '"ALL OVER MELTY BLOOD ~ Melty Blood Actress Again for Limited Edition Original Sound Track."' by '"来兎"'
[2023-03-18 18:46:24.804 -04:00] [INF] [38] Jellyfin.Plugin.Listenbrainz.ServerEntryPoint: Successfully submitted feedback for user "Meloom" for track '"Strange Dreamer (午前零時の逢瀬)"'
Appreciate all your work on the app!
Funny that you assume that the issue is on Symfonium and not the addon 
Anyway the way it works in Symfonium is also a proper way to handle all the cases.
Doing the thing to please that addon would break other more important features for Symfonium users. And could lead to duplicate scrobbling and playcount updates.
You should see with that addon what they do can do support offline scrobbling and so the PlayedItems endpoint.