You are probably facing a Navidrome default configuration issue.
By default Navidrome reports fake filename that does not take in account track number. So it’s possible that on some albums 2 songs report the same filename, and so Symfonium is smart and only offline cache 1 since it’s the same.
Next Navidrome release will have a fix (but will require you to sync again the whole library 
)
See Tracks with same name are overwritten when syncing for another solution but that will also require that you resync your whole library
TL;DR: Navidrome returns invalid data that leads to different files being seen as the same.