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:-Â
(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.Â
(1)Â Â Â Â Â Â Â Â Â Â How to back up your iPhone, iPad, and iPod touch - Apple Support
(2)Â Â Â Â Â Â Â Â Â Â Restore your iPhone, iPad, or iPod to factory settings - Apple Support
(3)Â Â Â Â Â Â Â Â Â Â Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
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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
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