WhatsApp has been working on a feature that will properly flag calls and video calls that you missed because DND was turned on.

 DND flag for missed calls is present in WhatsApp’s most recent beta in version number

Every time you miss a call because the Do Not Disturb mode is on

you'll see a helpful message immediately under the missed call history item stating that the call was hushed by Do Not Disturb.

person calling you won’t be able to tell if you had Do Not Disturb on or off

but if you want them to know you weren’t purposefully ignoring their calls, you can just send them a screenshot of the indicator.

This feature was first made available in the beta for version in September, but it was never really activated.