Green and flashing yellow light at same time??

At a four way and there was a green light but a flashing yellow above it (I was going straight) I’ve never seen that before so I just copied the person in front of me (they stoped and yield). I did the same thing since a truck drove out in front of me. However the cars behind me honked at me yet there was oncoming traffic 🤔 what does this mean because I see nothing on the internet about both lights at the same time? Both turn lanes had both red arrows so this was only for people going straight. My parents and friends also said they have never seen or heard of that before.