We had an image accidentally upgrade to 1803. We were unaware when the image was handed to us and attempted to build a new linked clone pool with a persistent disk and it failed to provision. The VMs comes up with the date/time set without the timezone (UTC 0) and eventually fail to provision with error "View Composer agent initialization state error (22): Failed to mount persistent disk. Please check the parent VM if an existing disk is using the same drive letter. (waited 0 seconds)". We recreated the pool without a persistent disk and the linked clones provisioned successfully.
I understand that 1803 is currently listed as a supported for 7.5 and not supported for 7.4 (Supported versions of Windows 10 on Horizon Agent Including All VDI Clones (Full Clones, Instant Clones, and Linked Clones on Horizon 7) (2149393).