Battery is draining after latest update ?

Your battery declines continuously. This is normal. It sounds like your battery has reached the point where it needs to be replaced.

Check the Battery Health as follows:

Settings > Battery > Battery Health > Maximum Capacity

If the Max Capacity is 80% or less, the battery needs to be replaced at the Apple Store or authorized Apple Service shop.

Find Locations - Apple

Phylscircus2 wrote:

There is nothing running in the background.

That is almost certainly incorrect.

Shortly after each iOS update, your iPhone needs to perform housekeeping chores such as refreshing search lists.

If your battery is “needing service”:

iPhone Battery Repair & Replacement - Apple Support