Welcome!
I had my iPhone 11 battery replaced about six months ago.
Did you have Apple replace the battery? After only six months of use, this:
my battery is not holding a charge when I check on its battery health it says 78% percent and it says your battery needs to be replaced.
sounds more like someone installed a non-Apple battery rather than a iOS upgrade issue. The last update of iOS 18 on my iPhone SE 2020 with the original battery changed nothing.
Have you tried restarting the phone to see if the values change?
Many after-market batteries are of very poor quality and fail prematurely. Are you getting any advisories about non-Apple parts being installed?