On Apple Fitness app, steps are no longer being recorded yet calories are snd the ring close.

Thank you!

It could be a sensor that has failed. I suggest as this is a top end iphone that you reach out to Apple Support directly. You should be able to reach them here  

Click here > Contact - Official Apple Support  

or phone numbers  

Click here > Contact Apple for support and service - Apple Support   or

Click here > Genius Bar Reservation and Apple Support Options - Apple 

Before that you could Backup, restore to factory, then restore from the backup:-  

(If the phone needs attention you will have to do this anyway) 

See here for the resources you need. Allow a couple of hours: 

How to back up your iPhone, iPad, and iPod touch - Apple Support 

Restore your iPhone, iPad, or iPod to factory settings - Apple Support 

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support 

Are the steps zero? That would be very unusual. In Health app, Browse data, how do the Steps look there?

I assume you have no Apple Watch as we are talking just the Red ring, yes?

For unusual glitches, Force restart the phone.

Click here >   Force restart iPhone – Apple Support (UK) 

We are having forum problems. Can I ask if you got an email notification of my replies, or did you use "My Subscriptions" to follow?

TIA