The game no longer detects older version ocean monuments. On my personal world I was making a guardian farm and when I updated to the latest snapshot the guardians stopped spawning. After looking into it further, on worlds created on 20w21a, the guardians do spawn, but not in pre-snapshot temples. I looked into it further and on my personal world I /located monument and it told me the nearest monument was 2000 blocks away despite me standing in one.