That's a little bug I still have to find out.
I have noticed around my location, if a signal goes from GREEN to RED, it shows YELLOW for a second like a relais falling down and hitting the contact point for YELLOW. I might lengthen the time for YELLOW showing so it isn't as much as flashing as it is now. How to remove it from RED to GREEN I still have to find out. In the ModelEditor it shows as it should be.
In this video the behavior I mean can be seen at 2:56 (red to green)
In this video the swith from green to red at 27:15