The problem may be in the transfer speed between hosts.

What could be the algorithm for copying the RAM of a running virtual machine to another host?

1. Copy the RAM of the running VM to another host.
2. While the copying was in progress, the RAM of the running VM has already changed.
3. Copy the changes.
4. While the copying was in progress, the RAM of the running VM has already changed.
5. Copy the changes.

Finally, we understand that this is an infinite loop.
Freeze the running virtual machine.
The RAM of the non-running virtual machine no longer changes.
Copy the changes RAM of the non-running virtual machine.
After copying the changes, the RAM of the non-running VM on the old host matches the RAM of the VM on the new host.
Unfreeze the VM on the new host.

The more uncopied changes at the time of freezing, the longer the freezing time.

Copying of uncopied changes after freezing cannot happen instantly.

It depends on how much RAM has not yet been copied to the new VM-server at the time of the freeze.

If a test virtual machine does virtually nothing, then there are not many changes in its memory.

There is always short-term freeze when migrating to another host.
How short is this short-term freeze?
If it is so short that no one notices it, then no one notices it...