I thought I'd share my story here as I've experienced this twice now.
I have a MBP 15" 2018 i9, purchased around July 2018. Using Apple charger that came with the laptop.
July 2019 - The first occurrence of swollen battery. This happened very rapidly, literally overnight. Below are two indicative pix. The lid would not close and the screw holes were stressed because the lower frame was stretched / bulging. I had taken it the Apple store where the first 'Genius' I dealt with said they will only replace the battery and then the frame will automatically 'unbend' itself - you couldn't make this stuff up. I had to explain to him that stretch marks and folds in aluminium don't automagically go back to normal after the stress has been removed, and I had to insist. Then they agreed to replace 'Bottom Case', 'Top Case' which is essentially the bottom part of the laptop - these are the terms they used. I asked about the now bent motherboard or chances that the charging electronics on the motherboard is what caused the issue, to which the response was, it will be fine and the motherboard is fine (without inspecting it).
28 Feb 2020 - Fast forward a few months and I've noticed that there is swelling again. Now it is not as bad as the previous instance but the base is again bulging so that it wobbles when placed on a flat surface. It's hard to get a good pic, but think of it as one of the 4 feet raised in the air. I had taken it back to the Apple store and after back and forth, they inspected, found it is a swollen battery and suggested a repair - again. To which I said that if this has happened twice in about 7-8 months, what if it is the charging hardware, so I can't keep coming back until apple care runs out and then I have to pay. They were very nice and agreed with management a full refund. Only problem was when they attempted to refund, there was an error on their system that prevented them from doing so. So now I'm dealing with apple care directly (which have a totally separate process and are going to do again what the store did - diagnostics, inspection etc, even though there is a note on the repair that a refund was agreed and second time swollen battery is the issue). It is a slow process. I'll drop in an update as soon as I get more information or it comes to some conclusion.
Bottom line is that I suspect there is a fundamental flaw in either the battery (and replacement batteries) or the charging electronics within the laptop that is causing this to happen. And if it happened once, it may happen again, as demonstrated here - so your laptop is now a fire hazard in your home.
🙁😡. Please keep it in a lower risk area of your home when unattended - Crazy that we have to do this!
Please, if anyone has had second (or subsequent) swollen battery issue after the initial repair, please let me know.
Just to add further context, I'm based in the UK and there are consumer protection laws here that may differ from other countries which forces the hand of manufacturers to protect consumers against fundamental flaws like manufacturing defects etc, hence the offer of the refund. I'm not sure Apple will do so in other countries. I'm not an expert on the topic, just relaying my understanding.
Update...
4 Mar 2020 - Apple has offered a full refund. Resolved.