Is this happening just for this printer?
Some printers have the habit to go into sleep mode after a period of inactivity, to save power. In such a state, the device only responds to arp requests (and other direct traffic) and will wake up again. If you enable mac-authentication on the port during such a sleep period, it will appear that the device drops off until it generates traffic again.
If you don't see an authentication request after enabling mac-authentication on the port, try disconnecting the cable (or disable port) and reconnect (or enable); as last resort reboot printer and you should see MAC authentication happen.
As when printers go to sleep, the authentication might time out and get disconnected, in the ArubaOS switches version 16.05 a feature called MAC Pinning was introduced to solve this issue by keeping the authentication active till the device starts sending traffic again. Check out: http://community.arubanetworks.com/t5/Campus-Switching-and-Routing/What-are-the-new-AAA-security-features-introduced-in-ArubaOS/td-p/412700
