AirTag Locations Out of Sync

That does sound like an odd situation - I agree that your AirTag's location should be updating uniformly on both iPhones. The fact that your iPhone 8 shows more recent locations while your iPhone X doesn't is indeed unusual, especially since they're tied to the same Apple ID.

Firstly, make sure your iPhone X has Background App Refresh enabled for the Find My app. Go to Settings > General > Background App Refresh and check that the switch next to 'Find My' is turned on. This allows the Find My app to update information in the background.

If that doesn't seem to resolve the issue, try signing out of your Apple ID on the iPhone X and then sign back in. You can do this from Settings > [your name at the top] > Sign Out at the bottom. Remember to back up any important data before you do this to prevent any accidental losses. Once signed out, restart your iPhone X, then sign back in. Check if this synchronizes the AirTag's location data across your devices.

If the issue persists after trying these steps, it may be worth reaching out to Apple Support for further assistance. To do so, simply select the "Support" link that is located on the bottom left-hand side of this forum's web page.

airtags does not have gps, airtags only means of communication is bluetooth so about a 30meter range

the way it works when further away from you than 30meters is that it can piggybag on iphones within 30meter range of it if they have bluetooth turned on and a connection to the internet, then the airtag send the location of the iphone within range to apples servers where your find my app can get it, if no iphone is within range of your cat then it's dead in the water

Oops, you're right! There isn't an option, specifically for the Find My app. Must not have had enough coffee this morning. Sorry about that! However, enabling app background refresh should, AFAIK, keep the Find My app up-to-date.