I would first suggest ensuring both fans are plugged in and rotating.
It could be a faulty battery or this may be normal behavior after an aftermarket battery is installed. Even though the fans are running in a normal speed/manner, I would still make sure the fans and cooling fins of the heat sink are clean and clear of obstruction and ensure the battery is not swollen. If the battery is getting hotter than normal to the point where it feels like or you know it is over heating, the computer should detect this and speed up the fans to compensate for the excess heat.
The parts from iFixit are generally excellent in quality and typically among the best parts available. Unfortunately, Apple does not provide avenues to obtain genuine or original parts for their devices. Sadly they take action to prevent third party repair on many levels and would prefer you buy a new device when you have an issue.
Did you notice a drastic difference in the weight of the original battery vs the new one? Any impact on performance since the battery replacement, good or bad? Lastly, does the battery perform as expected? As long as it isn’t getting overly hot and all is performing as expected I personally would dismiss the heat as normal behavior of an aftermarket battery. You can contact iFixit to see if they offer a warranty or return program to ensure you did not receive a faulty battery.
I’m sorry I don’t have a more direct answer to your question.
I hope this helps.
Thanks,