Why is LAN sync not working?

I'm seeing the same thing, have done for the last year. It's driving me nuts.

At the moment I have a MacOS desktop computer that's fully up to date, a Windows laptop that I just booted up which is a couple of weeks out of date due to being turned off, and a Windows desktop that I just connected to my Dropbox account.

My 64 GB Dropbox folder includes lots of photos and other large files which are perfect for LAN sync.

However, both Windows machines are downloading all the files from the Internet instead - very slowly, and eating up all my Internet traffic allowance.

I saw the laptop using LAN sync briefly, 2 days ago, but before and since it has used only Internet download. Both machines are estimating 12-20 days to finish their sync.

I've used Wireshark to look at the network traffic, and I can see all 3 machines exchanging LAN sync protocol messages just as you'd expect. However, no significant part of the file updates are being done via the LAN.

I've previously logged a support ticket on this issue and got no useful help at all.

Frankly I think this feature is a dud and I'm completely frustrated, to the point where I'd dump Dropbox for more reliable software if I could find it. Can anyone recommend an alternative that provides similar features, including robust LAN sync functionality?