You have not said what the flexible circuits are made from. However in general I do not believe 125 C is going to shock the circuit if the part was put into hot oven.

This is a very good question and most people have different views but I am an old flexible printed circuit board guy both polyester and kapton material sets.

The idea is to drive water absorption out of materials and it should a slow process of allowing the pcb to ramp to temperature and cool to ambient or close to it over 8 to 12 hours and the temperature should be 105 degrees C.

Naturally Kapton material sets can handle higher temperatures than polyester. KAPTON can handle 125 degrees C but the idea is to drive water out of the material set slowly and effectively.

What’s the end users process? Soldering the flex (lead free or not), Surface finishing, plasma processing, drying a high layer count rigid flex preventing shorts at electrical?

Typical Bake for soldering flex circuits and rigid flex 12 layer and below: 2 hours min @ 250 degrees F Solder within 2-3 hours after bake. No need for 6 hours if the fabricated has laminated at the correct temp and time.

Yes oven should be convection with air flow and at temperature.