It's definitely a bug. Dumb feature in my opinion. Especially if your on-call for work, or on the other line and can't receive calls. Imagine a doctor or surgeon's frustration with this. I hope a fix is in the works!
Sony Z5 premium
I'm having the same issue. Anytime I make or receive a call, DO NOT DISTURB automatically activates and I have to manually turn it off. It's super annoying especially when it deactivates call waiting. Any fix?