I’ll throw out a contrasting opinion. I tried a mare that was rock solid bordering on a bland personality. She had a flat affect, very much a “burned out lesson horse” vibe and was a push ride. During her PPE the vet noted she was obedient but shaking during flexions. You couldn’t see it from even a few feet and I remember thinking “eh, I wouldn’t love the vet either and she’s a good girl for standing so still despite being nervous”.
In reality, this is a horse with deep deep baggage who is highly distrustful of people. She was so shutdown and disassociated that she came across as easy. She did exactly as told because she thought she had no option. The vets presence pushed her threshold enough that some of that became visible. Unraveling all of that baggage has been tricky and in reality this is a hot hot mare when nervous.
I’d be curious to do a second test ride and push the horse just slightly. Maybe ask the seller if you can do a little groundwork or see how he responds to an umbrella opening or a tarp. If he spooks it isn’t a big deal, he’s a horse but you may see more to this horse with a little stress. The fact that he was labeled as hot and you tried him on an unusually hot day would definitely make me want to see him a second time before proceeding. If he’s got a seller may have lunged him down a bit and then you saw him on a warm day and that horse isn’t going to reappear again until June.