I believe you also need to get on the plane to trigger being removed from the later flight? The rules may be different for EP/CK with multiple bookings, but I cleared standby a few months ago into a middle seat towards the back. I told the agent that I didn’t want the seat and to give it to someone else. He was able to remove me from that flight and I didn’t lose my confirmed later flight (both were showing in the app).

I’ve always been unsure on this point, which is why nearly never use the standby feature.