its normal for these old engines. When you come to a stop at a light the ECU runs closed loop for a few seconds, then jumps back to open loop slightly rich and sits there until its time to go.
I noticed the same thing years ago. I took a video of it too, want to see...
I also made a thread asking "why, why?" and the only thing I can conclude is that either back in that day, narrowband sensors were unreliable at a stop (cold EGT?) Or, it had something to do with the longevity of the engine, perhaps running slightly rich at an idle helps keep the engine healthy long-term. Remember this is 90's technology so sometimes we have no real explanation for what we see.