Rapid battery drain & overheating home button
I bought a used iPhone 6s a month ago, the previous owner had replaced the battery just before selling. As I got the phone, it showed that touch ID couldn't be activated, the previous owner said he didn't have that problem before the battery replacement. I didn't think much of it until I started to notice that the battery was draining very fast, especially on standby. Also the home button gets hot when I'm not using the phone.
Yesterday I noticed my screen bulging up from the left edge, apparently the battery was swollen and I paid to get it replaced, only to get the new battery and experience the same kind of draining (for example currently it has been on airplane mode and untouched for 40 minutes and the battery went from 87% to 61%).
Could it be the home button/touch ID? And would replacing it work, or is there a risk that the problem would still persist and I would just waste money while the phone would still be practically unusable?