MID Server does not automatically upgrade. You may see a banner on the MID Server record similar to the following:
MID Server agent log shows the following every hour or upon restarting the service:
03/08/19 07:57:16 (072) AutoUpgrade.3600 Checking to see if MID server needs to upgrade.
03/08/19 07:57:16 (838) AutoUpgrade.3600 Installed packages are up-to-date.
Restarting the MID Server has been known to resolve this issue. Upon restarting the MID Server the upgrade completes.