I've noticed this as well. My 2021 Mazda does a great job of keeping the set speed, even going down hill. My 2021 Odyssey Touring, on the other hand, is terrible. I've also noticed the getting up to speed being a problem as well. I will get onto highway, accelerate up to 70, set the cruise at 70 then bump it up to 73 or 74, and then the car actually will start going slower, going down to about 60-65 before it starts to accelerate again back up to speed. However, I do think the downhill speed problem is more of an issue.