I have seen the event only the server really into sleep mode, which run on note pc.
Could be a hardware problem as well (You wrote that the Mini never goes to sleep mode according to system settings)..
Is the Mini one of the late 2012 i7 quad core ones? Those fast devices are in my environment not the most stable ones. We have two of them restarting... Searches (google) showed some problems with temperature, RAM, motherboards..
What says the system log?
Mac mini here as well, none of the settings allow the mini to go to sleep. This is about #17 of the minis that I've used over the years and I've never seen this issue. Same 182: FileMaker Server is going to sleep. The entire cache will be flushed. All schedules will be suspended
This mini is currently 10.12.1, 3 GHz 16 GB RAM, SSD.