I use freezer bags which are specifically made for food storage and will stand hot water (since part of the preparation process for freezing vegetables is blanching them for a minute or two in boiling water, they go into the bag pretty hot). Note that your rehydration (not cooking) water cools down the instant you take it off the fire, so will be well under 212* F (even at sea level) when you pour it into the freezer bag. Of course at higher altitudes the boiling point of water is much lower than 212*F/100*C.

I have never tasted any plastic taste in my food and strongly suspect that you used bags that were not meant for food storage!