HR at altitude. how does it differ from sea level?

ok, so I have done enough racing and hard training at sea level to know that I can press very hard and reach an observed HR of 165, and sometimes sustain that level for an extended period. for reference, the highest HR I have ever recorded in the last 2 years is 170( which is basically inline with a 220-age max HR calculation).  But at altitude, from 6,000ft where my house is, up to 9500ft where I train, my observed max HR is more like 155-157, achieved during races and very hard intervals.  for some reason I cannot get my HR into the mid-160s when I am at altitude.  Its also worth noting that I’ve spent all of this year working with an Evoke coach to cure my vicious ADS…and it has worked !!.  Note that my old race data from last year including max HR was recorded when I was inflicted with ADS and did everything anaerobically.  so 2 things have changed in my training: ADS has been largely handled, and I am at altitude now. given these 2 variables, do they explain why  can’t get my HR to 165~ like I used to? is this good? am I not trying hard enough ?  thanks