iPhone 16 Pro heating and fast battery drain after updating to iOS 18.4.1

After an update, your iPhone must work extra hard to run additional background tasks. Some of these include indexing which is creating and sorting a searchable database of your data on the new software. Also fully installing and moving new features to the right places. Which you can experience an increased battery drain and thermal temperatures. When the tasks are fully complete, which usually takes a few days to a week, your iPhone will return to normal conditions.

You should also read my User Tip which further explains this in detail:

Addressing the Common Increased Battery D… - Apple Community