Hi samyak35
welcome to the Apple (user-to-user) Community.
Re: "AirPods Pro mismatch after sending for repair"
Seems after replacing AirPods Pro earphones, the replacements may have a new serial number.
Apple support's online article (link below) may be helpful:
How to: Set up your replacement AirPods or charging case - Apple Support
You might want to check that your iPhone iOS has the latest update, so the AirPods firmware is kept up-to-date.
Eg: About iOS 16 Updates - Apple Support says the latest version is iOS 16.6
More info: About firmware updates for AirPods - Apple Support
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Re: "find my is no longer working. It shows Airpods mismatch error"
If needed: You can scroll to: How to add your AirPods ... to Find My in the article below
Set up Find My on your iPhone, iPad, iPod touch, or Mac - Apple Support
More info./ support:
AirPods Support Website:
Find all the topics, resources, and contact options you need for your AirPods....
AirPods Support - Official Apple Support
All the best :-)