Hey there!
I can see how this can potentially be an issue, because when you remove your phone from your ear, you want to be able to use your phone screen again.
Things such as protective cases and screen protectors could potentially obstruct the proximity sensor of the phone, but if you believe that you have an issue with your phone, you can reach out to Apple Support.
The US number for AppleCare is 1 (800) APL-CARE or 1 (800) 275-2273. If you are outside of the US, then you can use this link from Apple to get the appropriate phone number.
https://www.apple.com/contact/
There, a support advisor may offer to run Diagnostics on your device, or help better understand how the proximity sensor works on iPhone.
Hope this helps!