Windows 10 Offline Files - computer thinks its offline, when it's not.

Server is Windows 2012 R2. Clients are Windows 10.

VPN is a Watchguard SSL VPN. Users are connected on fast VDSL connections.

When Offline Files is enabled, users connecting via the VPN can no longer see any folders other than those already synchronised. File explorer shows the computer working in offline mode.

I have checked the network location, and this shows 'domain' as expected.

It appears that when connected to the VPN, Windows is perfectly happy to authenticate against the network, browse network shares it's never seen before, there are no speed issues, etc, but the minute offline files is enabled, Windows (file explorer only) thinks the computer is offline.

There is no GPO set to describe the slow speed threshold, so the default of 500kbps should be true. The connection is operating nearer 80Mbps.

I've set a GPO "Computer Configuration > Policies > Administrative Templates > Network > Offline Files > Configure slow-link mode" to disabled, which seems to have resolved the issue.

However, I'm more concerned that Windows believes the computer to be offline when it isn't, and I wonder if there's a firewall issue I should be aware of?

Any pointers?