AppSync 3.5 "Update a service plan" REST API call is disabling service plan

AppSync 3.5 "Update a service plan" REST API call is disabling the service plan and setting the planType to "UnusedPlan". This is new behaviour from AppSync 2.2.3, which we upgraded from. The REST API URI is "/instances/servicePlan::{uuid}", where the {uuid} is the service plan id.  The service plan id is valid and the XML payload has been verified that it is correct and is also the same as was used with AppSync 2.2.3 REST API. What do I need to do to enable this service plan and set planType to "Repurposing" instead of "UnusedPlan"?