Dry soil/drought probably helps a bit. But it’s clearly getting easier to get high end (95+) heat. The June big heat had notable H5 heights, but this one hasn’t as you’ve noted.
Yeah I think it’s probably just the way it models convection. It’s too enthusiastic with (dry) convecting air mixing the boundary layer during daytime heating. You don’t get that in the cool seasons. That’s my guess at least.