See If your iPhone, iPad, or iPod touch is lost or stolen https://support.apple.com/en-us/HT201472. And, see this heading If Find My [device] is enabled on your missing device. See step 6.
"Erase your device. To prevent anyone else from accessing the data on your missing device, you can erase it remotely. When you erase your device, all of your information (including credit, debit, or prepaid cards for Apple Pay) is deleted from the device, and you won't be able to find it using the Find My app or Find iPhone on iCloud.com. After you erase a device, you can't track it. If you remove the device from your account after you erase it, Activation Lock will be turned off. This allows another person to turn on and use your device."
Did you erase your iPhone. If you did, it appears that the thief was able to remove it from your device list. So, you need to change your Apple ID password.
See Change your Apple ID password https://support.apple.com/en-us/HT201355
And here is What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071
And see these security-related Apple Documents:
If you think your Apple ID has been compromised https://support.apple.com/en-us/HT201303
Security and your Apple ID https://support.apple.com/en-us/HT201303
iCloud security overview https://support.apple.com/en-us/HT202303
Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac