Using the Retention Strategy "Azure VM Idle Retention Strategy" with a Retention Time of 45 minutes, the Azure VM Agent log show that are sometime deprovisioned for "Unknown reason" before the idle Retention Time has elapsed.
In addition, the Cloud Statistics lists those agents with a Completed time stamp and an Operation time less than the Retention Time. This can happen even before the "Max Jenkins Agents Limit" has been reached.
Our agents are rebooted for cleanup every so often, and the cleanup seems to be happening while agents is rebooting. See attached Jenkins log jenkins-filteredAndScrubbed.log filtered for one of those agents and scrubbed of internal information.