I've been through the documentation (https://docs.microsoft.com/en-gb/azure/active-directory/devices/enterprise-state-roaming-overview) on setting up Enterprise State Roaming, and as far as I can tell, everything should be set up, but I'm just getting no results from it.
So, I'm trying this with a test user and a number of test machines. The test machines are all Hybrid Azure AD Joined, and were registered into Microsoft Intune by the test user
The machines all seem to be registered successfully:

The test user has a Microsoft 365 F1 license
and has been enabled for Enterprise State Roaming
When logging into a test machine, it appears that Sync your Settings is enabled:
Yet, when logging into a second test machine, the settings clearly aren't synching:
In the event log, I get a mixture of "Element Not Found" (8002802B) errors, and "The Request is not supported" (80070032)

To troubleshoot this, I've tried moving the test machines to an OU with no GPO's applied (other than those essential for connectivity, and to ensure ESR/Intune registration is enabled)
I've tried performing a full wipe and reset of the test machines from the Intune portal
I've moved the machines to a VLAN on the 'outside' of my firewall
Any ideas on what might be preventing these accounts from synching? Or what tests I could try next?
Many thanks