Bumping this topic.
I'm experiencing the same issue as OP but I'm noticing it more and more lately.
When I set to High, it blows all-out to produce as much warmth as possible, as you mention, but when you set the temperature down to a few degrees lower, it automatically produces cooler air. For example, @ 79 degrees, for example, down from HI, it should still be blowing sufficient warmer air but I feel cooler air. I think it's blowing cooler air to try to lower the cabin temperature to 79 degrees, down from "HI" and then maintaining the cabin at 79 degrees that is set, blowing either warmer air or cooler air. Keep in mind the "Auto" isn't on and I can expect this to happen if the automatic climate control is on, but it's off. I expect that if I lower the temp from HI to 79, I would still get slightly warmer air, but I feel cooler air, and then I would have to bump it up to HI again to get warmer air.
Anyhow, I'll bring this up on my next SC appointment.