icloud storage

You are looking at iCloud Storage and not iPhone Storage. By default, every Apple ID has 5GB of iCloud Space over and above the iPhone Storage.

Look closely storage of iPhones and iCloud.

iCloud storage is primarily for keeping a copy of your iPhone data safely on the cloud. If you delete data on iPhone, data on iCloud is gone too.

Turn iCloud features on or off

1. On your iPhone, iPad or iPod touch, go to Settings > [your name] > iCloud.
2. Turn iCloud features on or off.

Turn iCloud features on or off - Apple Support

iPhone Storage (Setting > General > iPhone Storage)

1. Total storage remains the same throughout the life span of the iPhone
2. Storage can not be enhanced
3. You may take back up on Mac or PC using iTunes without iCloud

iCloud Storage (Settings > [Tap your name] > iCloud)

1. By default, every user gets 5GB of free storage space
2. You can buy more storage based on your requirement of 50, 200GB, 2TB with a monthly subscription
3. Based on your storage capacity you can back up iPhone or iPad
4. Space can be shared between your family members for a storage plan beyond 200GB

the icloud subscription follows the appleID if you changed that when you changed phone that would explain it

otherwise if it was me then I would reach out to apple support to have them sort it out

https://getsupport.apple.com/