Well I guess I am not the only one with swollen battery problem on my 2018 MBP (although the original OP's is 2015 model). Just noticed the lid was not closing tight. Both corners were slightly raised when closing my MBP.
I used mine as a portable computer 80% of the time and due to COVID-19, I hooked it up to eGPU and used it with 4K AOC curved monitor.
It does seem like when it is constantly plugged in, the battery seems to swell.
Fortunately, I purchased AppleCare+ (I do with laptops, but not for MacMini), so it is covered. Worth the cost of AppleCare+ for sure. I just got the FedEx box today from Apple and I've sent it in already. Hopefully, they will replace the battery tomorrow and ship it right back out. Fortunately I have MacMini at home also.
I guess I have to remember not to keep it plugged in all the time.