Automate performs a check if Premium has been purchased when the service running the flows is started, and whenever the flow list, flow details and settings pages are shown.
Automate doesn't explicitly hold a connection to the Google Play store app, the
"billing library" we developers are forced to use manages that internally.
Generally such connections should not prevent the app they connect to from being killed by the system when memory gets low.
But if available memory is plentiful then such connections may remain to increase performance by not having to restart the app every time the connection is accessed.
How/when the billing library accesses the connection is not documented, Google doesn't even release its source code.