Thumbnail display issue when Thunar has been running in daemon mode long enough

The title is imprecise but I can't reproduce the problem reliably. However, I've been noticing it for a while, typically after starting my Xfce session (which runs thunar --daemon):

• new files don't have a thumbnail (typically new downloads made since the start of the session, without having yet opened Thunar once);
• reloading the view has no effect;
• Tumbler's logs show that no requests are sent to it.

Running thunar -q then restarting Thunar (in daemon mode or not) solves the problem.

I hadn't opened an issue until then because a lot of changes were taking place about thumbnailing, but now that !416 (merged) has been merged I think it's time.