Well, I had a drink or two with Mr. Jameson. And he smoked one of my cigars, I smoked the other. Here's what we came up with.
Something may have changed during the R&R of the safety valve, but we can't know for sure. The fact that the valve will open when thermostat is in broil tells me valve is OK, now.
If the pilot changes from broil to 550 and oven temp is below 500 indicates a bad thermostat. It thinks oven temp is high, therefore it retracts pilot flame to allow thermocouple to cool and close valve.
I don't recall ever seeing a thermostat with different pilot flames for bake and broil, and that doesn't make sense. Safety valve is either open or closed.
It is possible that the thermostat has an "off mode" pilot flame, a "ready mode" ( control on but not calling for heat, cycled off, satisfied) and a heat mode. If this were the case then "ready mode" is what we are seeing when thermostat is set below broil. Again this indicates bad thermostat.
So, That is where I believe we currently stand. I sure hate to "shotgun it" and just throw parts at the problem till it goes away. However, It may be a good idea to replace safety valve anyway since it has been opened up, and it is a "safety". Replacing the pilot makes sense as well. Your already in there and they do corrode and degrade over time and they need to be reliable. But I'm afraid the real bad guy here is the thermostat.
OK, I hope that made sense.
McDave