In researching the topic of plastic and food safety, I found this
interesting article that discusses which types of plastics are considered safe and those that might leach BPAs, phthalates or other nasty things. Most sandwich bags (Ziploc, Glad, Hefty, etc.) are made from low density polyethylene (LDPE), which is considered a "low hazard" product not known for leaching contaminants. Elsewhere I found that LDPE is also resistant to most acids (including carbon dioxide) at temperatures up to 140 degrees F. So, nothing is guaranteed in life, but I suspect the bags are fine in this application.
The light baggie on the carrot jar expanded to a certain size and then stopped. I'm assuming even a small amount of internal pressure overcomes the seal created by the rubber bands. The baggie most likely expands because it is lighter and looser than the heavier plastic bags, which have shown no signs of inflating. That's all fine. If gas inside the jars is under just a tiny amount of pressure, there is very little chance for oxygen (or mold) to get inside and cause problems. Having the gas seep out means there is no need for me to "burp" the jars.
All good, since my wife gets annoyed when I check the jars five times a day. "They are going to be there for a month, no need to play with them every day!" she says....