I started a world in 1.17, I went to the end, I updated the world to 1.18 then I wanted to go end raiding, I could remember the approximate area of the stronghold so did a locate when I got there, but it wasn't there. I searched for a different one in another area using locate and that was also missing. I went in spectator mode both times to search. The attached pictures are of the area where the 2nd stronghold should be, from different angles