Why does SyncThing claim a dir has 9.5GB when dua reports 1.04GB?

[Nummer378 (Max)]

This is often caused by sparse files. See

Very wrong folder size in BOTH global and local state

Hi, I have just setup Syncthing on a new computer and added a new folder. However, the local state (and global state, but no syncing has been done so far) show almost 400GiB as the size of this folder while the real folder is about 20GiB. I have checked the folder size multiple times, there are (as far as I can tell) no weird symlink things going on) and as a funny bonus I know the shown folder size cannot be correct because my hard drive is not big enough for it. What is going on ? And importa…