Polypropylene (PP) is microwave oven safe. It wouldn’t be used for microwave heating applications otherwise. It is resistant to corrosion and chemical leaching. Resilient to most physical damages, impact and freezing. If plastic box is marked as microwave oven safe, then, it is 99.9% PP, the plastic number 5.
If it is microwave oven safe and resistant to heat. Then, why the ketchup is leaving a mark on bottom of this PP lunchbox?
Microwave heating works only if the object inside the enclosure contains water molecules. No heat is produced if water is absent. A simple test to know whether a vessel is microwave oven safe or not is to half-fill it with water and heat it inside a microwave oven for few seconds. If the water is relatively hotter than vessel, then the latter is microwave oven safe.
Explaining it deeper. In convectional heating, heat is transferred from the source to metal vessel the to the food. Microwave heating works the other way around. Heat is only produced when microwave energy comes in contact with water molecule. Hot polypropylene box is a result of heat transferring from water to plastic material. PP can resist temperature up to 275 °F or 135 °C. Above, it may react to the food it contain. It melts at 175 °C.
If the microwave oven safe material is pushed to its limits. Then, it may not be called safe. Safe is only safe within its own limits.