iPhone 6s batch with battery swelling!

I run a phone store in Ireland and have been encountering a large number of swollen batteries in iPhone 6s that I have sold lately.

I replace battery in each phone before sale but I have had 4 6s back in the past week all same screen has rose up due to the new battery swelling.

There is no water damage in these devices and seem to last about a week to 10 days before swelling up.

I have not changed my battery supplier but could this be the issue, a bad batch of batteries?

Could it come from the chargers i use to charge these for the 1st time (genuine apple plug and cable)

Very odd and just wondering has anyone any ideas or simply down to the batteries I have bought in?