Notifications sent after ESA reassessment from SG to WRAG?

One of my clients had his 2nd ESA medical and was moved from the support group to the work related activity group.  He only found out when he received an invitation for a WFI and when his money went down.  He did receive a letter telling him that his arrears payment had been put on hold because he might have been overpaid, but no decision letter like the one you get when you fail a WCA, or not even a letter notifying him of his new ESA rates. DWP informed the local authority of a change to his benefit (which resulted in a suspension of HB/CTS).
I had the same thing happen to a client some time ago.

As I actually never saw any decision letter following an ESA reassessment I now wonder whether these 2 clients were just unlucky or whether there is a flaw in the system?  Are claimants actually receiving notifications of ESA reassessments when these don’t result in total loss of benefit?