Mine has the random 'harsh' shifts the OP reports - I think the 4-5 is the most common one on mine, and with normal or slightly above acceleration into traffic. Not harsh enough that I think it is causing damage, but harsh enough to be annoying.
So far, I also can't produce them at will. It has about 20,000 miles, but has done it regularly since new.
I've not been able to correlate it to trans temperature (but I rarely see 180 on mine)
Did take the dealer tech that does the Allison work (and drives one of these) out -- but of course it didn't do it. As I'm writing this I wondering if that was because we were driving in the city, so speeds may not have been high enough to get 4-5 occurring at a high enough rpm for it to happen - time for some experimenting.
None of the previous dmax/allies I've run did anything like this.
2018 GMC 2500 L5P CCSB 4x4 Denali - Mineral Metallic; previous '11 LML, '10 LMM and 04.5 LLY