Why did face tag disappear in iPhone 16 Pro Max with iOS 18.4?

Well, the things I suggested were for a Mac-- sometimes I forget what I'm doing. You should try a Forced re-start,

Force restart iPhone - Apple Support

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after  the Apple Logo appeared indicating a re-start. 

Warewolf73 wrote: … I will try & store some elsewhere & see if that solved the problem, if works add more gradually

Well, it doesn't really give up, but 9000 may take more time!

If you have time to mess around, here are some things to try:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

Those are easy and quick things to try first. Then try these:

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it
• If you don't use iCloud Photos, then try rebuilding your Photos Library--close Photos and re-launch by option-command-clicking the app icon. iCloud with or without "Optimize" and shared Libraries, may make rebuilding more complicated.

I forgot that you mentioned the location maps-- are they all disappeared? Let us know what happens…

One problem with the most recent update on some iPhone models is, that the faces and locations will be hidden, as soon as we enable the Settings > Apps > Photos > Require Face ID.

Try to turn this setting off, if it is still enabled and see, if this will bring the locations and faces thumbnails back. It may be a a bug, or a design flaw - but Photos seems to be hiding the locations and faces as well, when we try to protect the hidden photos by requiring Face ID before they are shown.

With an upgrade, Photos re-scans the pictures, and it can take a while. I'd say, hang on for a week, and let us know how it has turned out.