That’s good. I am not saying Syncthing can’t be part of a backup solution, but it lacks a lot of characteristics a dedicated backup tool needs (because it’s not designed as a backup tool). See e.g. this thread for a possibility: [Project] Syncthing as backup solution
And I am not saying the problem isn’t connected with Syncthing, I am saying it’s probably specific to your setup. Anyway, that doesn’t really matter until there are actual hints as to what happened.
There is no single place for logs, it depends on your system and how you run Syncthing: FAQ — Syncthing documentation
Sure, versioning needs to safe the data somewhere, so it needs disk space. That’s one thing that many backup programs can do better, by storing backuped data at block level.