Altitude dip during wp delay and loiter delay, bug or feature?

I set in mission mode, auto mode, send the copter to an altitude using waypoint or takeoff. When I add a loiter delay or a delay for the waypoint, I realised the quad will dip 0.5m.

I tried it in sitl simulator, seems the same I thought its simulator issue. When I tried it on actual flight its the same.

Is this a feature or.a bug.