My guess would be that the cloud is combustion products from the engine start transient of the upper stage engines, vented through the ports in the interstage hot staging ring. Here is a video of a Raptor 2 static firing:
At the moment of engine ignition, there is an irregular flash of gases before pressure builds up to the steady state level and stable combustion is achieved (which doesn’t produce an obvious plume). Maybe that’s what caused the ring-like cloud. The launch was relatively soon after sunrise, so the plume, fully illuminated by the Sun at its altitude of 73 km, would be particularly visible against the darker sky.