Not a bug. It was a decision to how they wanted it to behave I believe.
I think they wanted to show that the 7 was faster than the 6S and all previous physical home buttons so they made it instant response. Initially it caused a few hiccups with the multitasking animation but they smoothed it out by 10.3.3.
Enter iPhone X. Now they might be wanting to show that swipe to up is the fastest way to close apps and interact with iOS which it is because they delayed the home button again with all the capacative home buttons.
It wasn't a bug. They wouldn't keep it that way all the way throughout iOS 10. I'm still on 10.3.3 on my 7 and I've compared it to an 8 at the store. My 7 is so much faster with interacting with iOS going in and out of apps. The 8 felt so herky jerky. It's night and day. No way I'm upgrading. And definitely not upgrading to an iPhone 8/8+. I would go for the X to have the instant response again but I don't have the dollar bills for it right now and my 7 is working perfectly.