I wonder if its not to do with some untested hardware issue since apple using intel processors seem to have changed regularly the inners, things less current when under powerPC ( I still have 2 of them...). I am trying to figure out what in high sierra can make my powerbook pro of 2011 over heat... ( because I burnt my motherboard (graphics...) beginning July ) or go berserk from time to time, I have just assembled a new one but this time a late 2011 motherboard after buying a 2012 for later usage, and again it reaches over 40 C after 15 minutes not doing anything special, the 2012 is already in mojave but so far not much time to play with as still sick and just finished a 3 weeks antibiotics treatment... but into as I dont know anything about the inners of the Mac Pro and mac mini but if like the laptops I would not be surprised... Not insisting for now as we have a heat wave passing just now and was 34C yesterday ( last time it was 36C here...) Monday 25C announced so I will try again...
AS now Im using my wife's ( my ex-hospital usage laptop...) Macbook air which is now under Mojave and am surprised that it stays cool doing exactly the same thing and yes it does heat when I start heavy usage but never like the Macbook pro which can exceed 45 after only 15 minutes when I use it seriously...
My idea was to investigate on the heating control, could it be faulty for my old HW and so not reacting in time? as like you before burning it did crash a few times without to me any reason and I could not see anything in the logs explaining except a panic but can't find the cause...