Anyway the resolution could have changed on the Mini?
Can you/have you, rebooted the Mini?
Is the Mini going to sleep?
See if it's related to this...
The 2012 macs (and later apparently) not waking normally from sleep after hours being in sleep. (noted here ...
"Why 2012 iMac/2012 Mac Mini won't Wake After Hours of Sleep (Hibernates/Powers Off)")
The sleepimage file still reappears even if never slept. Delete it (hibernate off, etc) - within minutes its back.
A note on that here - http://www.xlr8yourmac.com/feedback/Chameleon_SSD_Optimizer.html
About iMac sleep...
Try this: "sudo pmset autopoweroff 0" and "sudo pmset standby 0"