Unable to Connect to World error on all school PCs

I've recently deployed Minecraft Education Edition 1.18.32 across our school. Previously it was a mixture of pre-v1.18 versions ad-hoc installed under previous IT management.

Since the upgrade, nobody has been able to join other students worlds.

1. A student will open their world by click Host.
2. Press ESC to see the join code
3. Another student on the same subnet opens MinecraftEE, clicks Join World, enters the join code and click Confirm
4. "Is this the world you were trying to join? (correct info and user displayed) Click Confirm
5. Unable to connect to world.
   

It doesn't make a difference where in the school the student PCs are. It even occurs on the same physical switch.

All accounts are on the same Microsoft 365 tenant and licensed under the same 365 A3 agreement. Issue occurs between students which are on the same domain name, or with teachers which are on a different domain but on the same tenant.

All the PCs are Windows 10 Pro or Education 21H2, domain joined.

I've tried connecting via the join code, connection info code, and via the share URL methods, all with the same result.

I've checked our firewall (Palo Alto PA-460) and there are no connections being denied from the PCs I'm using to test.

Our endpoint protection (Bitdefender GravityZone) is set to allow all connections for the MinecraftEE executable. But I have also tried disabling it completely.

What do I look at next as a potential cause?

Is there some kind of log file produced by Minecraft which could help shed light on the cause?