It's an Android permission issue. Put it into /sdcardX/Android/data/com.nutomic.syncthingandroid/files/ and Syncthing can write to the folder just fine.
Nextcloud has the exact same SD limitation of needing an app-specific folder. For syncthing it's Android/data/com.nutomic.syncthingandroid/files/ and for nextcloud it's Android/media/com.nextcloud.client.