Low data mode
I updated to IOS 15.3.1 a few weeks ago and I only realised now that it doesn’t keep the low data mode enabled, I switch it on and as soon as I exit settings, it disables it.
iPhone 11, iOS 15
Welcome to the Apple Support Communities.
Restart your iPhone - Apple Support and try again.
It may also help to review: Use Low Data Mode on your iPhone and iPad - Apple Support.
If you need more help, contact Apple Support.
Jack