Title says it all. If soup (lentil-tomato in this case) is bagged after cooking and cooling then frozen, and later gets microwaved and boils inside the Zip-Loc baggie for a time is any nastiness from the plastic bag compounds likely to get into the soup?
There’s not much proof that it’s unhealthy–at least to digest the food. However, the heat of the food could melt the bag, and/or burn you if the bag opens or breaks as you are removing it from the microwave.
Personally, I prefer to use microwave-approved plastics when heating up foods, just because it’s less likely to melt if the food gets really hot. If I have food that is frozen in a bag, I will remove it from the bag and put the frozen lump in a microwave-safe bowl before heating it up. I have to put it in a bowl to eat it anyway, and it’s safer to put the frozen lump in than to try to have to pour hot soup from the bag into the bowl.
“Soup in a bag, Fran? Soup in a bag?”