"Greetings. This is Jeff from OnePlus Bug Hunters Team, thanks for your feedback.
This issue has been forwarded to dev team. If there is any update, we will keep you posted.
Thank you and have a nice day.
High regards,
OnePlus Bug Hunters"
I found this in the OnePlus forums. It is a reply to the exact issue you mention. Out of curiosity, if you bump the alert slider to vibrate or mute just during a call does it mitigate the issue? This is not the most elegant solution, but might work. I will try it myself if I get a call.