coder machines not shutting down in the expected manner after user inactivity

How is "inactivity" detected for coder instances? We have been working to reduce costs of coder instances and noticed that in some cases, coder machines will not shut down when we expect them to. What constitutes "inactivity" as described here: https://coder.com/docs/user-guides/workspace-scheduling and is there any guidance on how we can debug what prevented an instance from shutting down?