If you mean the system data is back to normal expectations (8GB + maybe 6GB) then all is working as expected after a big changeover.
If it comes back again permanently First try a Forced restart
Force restart iPhone – Apple Support (UK)
If no better
Backup, restore to factory, and test, then restore from backup:-
(If the phone needs attention you will have to do this anyway)
If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails, or you need to ditch the backup and start as new.
(1) How to back up your iPhone, iPad, and iPod touch - Apple Support
(2) Restore your iPhone, iPad, or iPod to factory settings - Apple Support
(3) Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
After that reach out to Apple Support directly. You should be able to reach them here
Contact - Official Apple Support
or phone numbers
Contact Apple for support and service - Apple Support