Those photos / videos aren’t defective in any way. They are in the same folder as other photos, have the same name, similar properties. I force-copied them into syncthing folder where they should be but syncthing didn’t accept them (probably because synced files have hidden flag in windows and those i copied in didn’t have the hide flag).
Anyway, i just did a bit of googling and arrived to this post where the guy has really similar issue. At the time of writting this comment, i already moved out the manually copied files so the syncthing folder should be in default state as it was before i noticed this issue. But now, after i restarted the syncthing on androind device end, it shows out of sync those 103 files i’m missing on my NAS server. I read in mentioned post that i should try to add flag “–reset-deltas”. Well, i’m able to do it on NAS side but unable to do so on Android side as the Web UI probably don’t have this feature / button implemented yet. On the other side, in this post a guy somehow found the button in the Advanced tab of Web UI. I was unable to find it.