Where exactly are you seeing this disappearance and reappearance?
Are you talking about your device list? The only way it could disappear and reappear from that is if your account is compromised and somebody keeps on removing it and signing back in again with your account information.
Read this document if you think your Apple Account has been compromised. --> If you think your Apple Account has been compromised - Apple Support
For iOS 15 or earlier, if you want to see if anyone else has access to your device or accounts, click here --> Checklist 1: Limit device and account access - Apple Support
For iOS 16 or later see how Safety Check on iPhone works to keep you safe. Safety Check for an iPhone with iOS 16 or later - Apple Support
Use the information in this document to check your Apple ID device list to find where you're signed in --> Check your Apple Account device list to find where you’re signed in - Apple Support
Related materials:
Personal Safety User Guide
Personal Safety User Guide - Apple Support
> open the Table of Contents and review the articles
A document with general information about security and your Apple ID --> Security and your Apple Account - Apple Support
Contact Apple for help with Apple ID account security. This page provides country-specific Apple Support contact information ➞ Contact Apple Support - Apple Support
If you are talking about the Find My service, what you see on the map will depend upon the device's ability to connect to the Internet.