The best and probably the most suitable reason could be that those batteries were not genuine ones. There are many of them in the market.
It's always difficult getting the original ones, and even if you find the genuine ones, they are always very expensive. Over heating was common with Samsung phones, I back then in university, I used S4. I cannot but agree more with you. I had to use power bank then to support it. Mine was mainly over heating but didn't swell because I had bought the original battery (very expensive).
In a nutshell, it's all about the quality of the battery.
Thanks for the read☺️
Cheers 🥂
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