If you've updated to iOS 15, make sure your AirTag is still in Find My
An increasing number of users are reporting that AirTags are no longer in the Find My app after updating to iOS 15.

Some users report AirTags disappearing from Find My after iOS 15 update
A bug in iOS 15 appears to remove the AirTag or make it unable to appear within the Find My app. Some users report success in re-adding the AirTag after performing a factory reset on the device.
AppleInsider recommends AirTag owners go into their Find My app and ensure they still appear there. Any AirTag that is missing will need a factory reset to be re-added to Find My.
The issue hasn't been widely reported, but since it requires the user to look for the AirTag in Find My, it could be a largely unnoticed issue. However, the bug can affect anyone who updated to iOS 15.
And, it's better to check before you need to rely on the AirTag to find a lost item.
Apple has released the beta for iOS 15.1, however, the company has been known to release hotfix updates without betas. An iOS 15.01 update could happen at any time to address reported bugs, like another bug dealing with storage reporting.
Read on AppleInsider

Some users report AirTags disappearing from Find My after iOS 15 update
A bug in iOS 15 appears to remove the AirTag or make it unable to appear within the Find My app. Some users report success in re-adding the AirTag after performing a factory reset on the device.
AppleInsider recommends AirTag owners go into their Find My app and ensure they still appear there. Any AirTag that is missing will need a factory reset to be re-added to Find My.
The issue hasn't been widely reported, but since it requires the user to look for the AirTag in Find My, it could be a largely unnoticed issue. However, the bug can affect anyone who updated to iOS 15.
And, it's better to check before you need to rely on the AirTag to find a lost item.
Just updated to iOS 15 and my AirTags were deleted from FindMy. I did get an alert I was being tracked by an AirTag so at least I got to see that side it. #Apple #iOS15
-- Josh (@JilsonW87)
Potential bug I found? The update seems to have wiped my AirTag devices. @AppleSupport @appleinsider #iOS15 pic.twitter.com/0LhZJPVrg4
-- Nathan Cunningham (@bkelevate)
Apple has released the beta for iOS 15.1, however, the company has been known to release hotfix updates without betas. An iOS 15.01 update could happen at any time to address reported bugs, like another bug dealing with storage reporting.
Read on AppleInsider
Comments
I was wrong. Found my AirTags listed in a different section of FindMy.
Mine are fine.
1. Reboot the iPhone (*sigh*, I know)
2. Open Find My and find the AirTags still missing
3. Go to the Apple ID security settings on the iPhone to reassure it that Two-Factor is still on (I knew it was, but the iPhone didn't believe me until I opened that settings screen).
4. Go back to Find My and marvel at the fact that the AirTags have reappeared.
Hope this helps anyone.