Wow. Replies from two developers after 20 Minutes. Respect!
I just tried restarting the Syncthing service (I guess that’s what you meant by “restarting”?), but it carried on like before. So I think you’re right, Audrius. Once it’s in motion → rien ne va plus. 
The “copier” action seems to be described differently in this thread, but I’ll test it out as soon as I can.
Finally, Jakob’s suggestion about using multiple folders might be a good workaround and I’ll look into it. Is there a theoretical/practical limit to the number of folders?