1.18, of course, generates worlds with a different set of strongholds from that of a world created in a previous version.

The bug:

If a world created prior to version 1.18 is upgraded, /locate and the Eye of Ender will direct the player to one of these new 1.18 strongholds - even if the chunks that this 1.18 stronghold would generate in are old chunks that never had a stronghold.

Reproduce:

1. Create a Creative Mode world in version 1.X that generates strongholds, entitled w1.X.
2. Copy the world and name the copy w1.18.
3. Open the w1.18 copy in version 1.18 and use /locate or an Eye of Ender to find the nearest stronghold. Copy down the coordinates of the stronghold. If the stronghold is entirely below Y:0, delete the worlds and start over from step 1.
4. Open the original w1.X world in version 1.X and go to the coordinates you just copied down, loading in those chunks. Verify that there is no stronghold at these coordinates.
5. Finally, upgrade and open the original w1.X world in version 1.18. Use /locate or an Eye of Ender to find where the game thinks the nearest stronghold is.

Expected result:

The game locates the nearest stronghold that actually exists, likely in a newly-generated 1.18 chunk.

Bugged result:

The game points you towards the location of the stronghold that you found in the copy w1.18 in step 3, but does not actually exist in the upgraded w1.X chunks.

This is a terrible inconvenience for people who have survival worlds that they've extensively explored in versions prior to 1.18, but didn't bother locating a stronghold in until after upgrading said worlds to 1.18. I spent several hours attempting to locate just such a stronghold, in a world which I can attach if desired.

I have attempted to reproduce this bug with other generated structures, but so far I can only verify that strongholds are affected. If more info comes to light I will update this report accordingly.