I would say that this is working as designed. I don't see a way for the REST programmer to trigger a cache reload.
In looking at the programmer's guide contained within the SDK download:
https://www.devconnectprogram.com/site/global/products_resources/avaya_aura_session_manager/releases/8_1_2/index.gsp#tabs-product-downld-4 https://www.devconnectprogram.com/fileMedia/download/eda3759e-3cc2-432c-91d0-8a53a9ae096
“The registration cache is a volatile cache. If the GUI is not accessed for 30 minutes, the cache is removed from the System Manager memory. However, if the GUI is not left idle, the cache gets automatically reloaded in every ten minutes. It can also be reloaded when required by an administrator by pressing the refresh button on the GUI. Pressing the refresh button on the GUI resets the 10 minute timer. The REST client developer must be aware of this when a request is made. For certain cache states such as when cache is not loaded or the cache is in the process of reloading, REST request cannot be processed. In this case, the REST client receives a 503 - Service Unavailable reply along with an error message. The Retry-After HTTP header will be set with an estimated wait time.
The wait time is estimated based on last cache reload time or size. Reload time is a function of number of Session Managers, Users, and SIP Registrations. On a small system this is usually 15 seconds. On a high capacity system this could be in minutes. Minimum reload time is 10 seconds and maximum is 120 seconds.
A very large SM user configuration can result in 300 - 600 MB cache on System Manager.”