@jason Looks like a bug to me. Either a code bug (where we should update it) or a doc bug (where we should note that the lastUpdateInstant isn't changed).
Can you please file an issue about this: https://github.com/fusionauth/fusionauth-issues/issues and we'll slot that into our work queue.
For a workaround, the only thing I can think of is to listen for the user.deactivate event: https://fusionauth.io/docs/v1/tech/events-webhooks/events/user-deactivate
You could create a custom user.data.deactivateTimestamp and set that when the webhook fires.