Random restart after battery replacement from ifixit

So in July i ordered a new battery from ifixit after my original battery had like 70% battery life. The phone worked like a charm for like 2-3 months but then it happened almost every day, that my phone just shut off and restarted with enough battery left (ca. 40-50%) and would restart with showing 10% charge. I asked ifixit for a battery replacement and they send a replacement (thank you). The battery came in october, i replaced the old ifixit one and the problem seemed to be fixed. Just now the same problem reappeared again, it had around 40% charge, turned off, restarted and it showed that the battery was charged only 10%.

Can anyone help me? Did i accidentally mess up my repair (doubt it though)?

Id like to add, that the phone in rare cases gets fairly hot, for example when scrolling through instagram reels. This is just additional information cause i doubt it has anything to do with the battery and more with the fact, that this phone is 4.5 years old.

Thanks for any input