Most likely that is what happened, I left them some times after charging and did not use the drone right away.MM batteries are 2x 18650 cells... I've never heard/seen of a case of those cells bloating. Unless DJI are not making them from conventional methods to reduce weight (conventionally made with metal cylinder walls).
Edit: many-a-threads of how to care for Lithium based cells.
Don't over/under charge them, store them between 40-60% capacity.
Keeping them constantly charged "ready to fly" at 100% will increase the chance of pre-mature cell degradation.
The auto-discharge feature prevents that from happening. It is more likely simply the age of the batteries. My oldest M2 batteries are now starting to show similar bulging.Most likely that is what happened, I left them some times after charging and did not use the drone right away.
Follow these tips:how can I avoid them from bloating ?
Do not fly with them!both of them are like this
A slightly swollen cold battery will swell massively when in use, as it generates heat, further expanding the gas inside that is creating the bulge. I have noticed this on the swollen one I have, when I use it on the ground, for adjusting settings and acquring satellites, before swapping it out for an unswollen flight battery. Would never use it for flight. It almost completely pops out of the clips when it gets hot.Do not fly with them!
Even if swelling subsided they are unsafe. I had one swell during flight (fortunately did not dislodge and disconnect). However on that occurrence I launched with a reading of 100%, but within 10-15 seconds got a critically low battery warning and auto land started. Fortunately I was still very close and low altitude. Found slight swelling of the battery then... much less than in your photo. Of note. I had inspected the battery at that time before installing and noticed no swelling.