As I mentioned we have removed all 7.4 Horizon view components and reinstalled 7.3.2, all our pools have been working fine since going back and with no issues experienced at all. My feeling is the issue is within the connection servers (Brokers), possibly the Adam database. At this stage we will be staying with 7.3.2. Also I do agree totally with you, the next step will be a support ticket.
Linked and Instant Clones worked like a charm after update 7.2.0 -> 7.4.0.
Just had trouble with a manual clone that got updated with Agent 7.4.0. VM was in state "waiting for agent". Workaround was:
1.) Remove UEM Agent, App Volumes Agent and Horizon Agent
2.) install the Horizon Agent again. Then UEM and AV agents.
3.) Afterwards I had to remove the VM from the pool.
4.) Power off the VM.
5.) Re-add VM to the pool.
6.) Power on the VM.
VM then was available again.