The bug

I play an old world (generated with 1.11, updated to 1.12 and now to 1.13, seed 6214752069859396573). I previously visited a stronghold that I found via /locate structure command, but had not marked visibly (close to coordinates -151, ~, -1414).

When attempting to revisit, the "Eye of Ender" and the /locate structure command bring me to the coordinates (-232, ~, -1592) suitable for my seed as would be generated in 1.13. I had visited both chunks in question previously, so no new stronghold was actually generated. The 1.13 coordinates are close to the "real" stronghold.

http://chunkbase.com/apps/stronghold-finder

shows the same coordinates as /locate structure. When selecting 1.9-1.12 as seed version in the stronghold-finder, the coordinates of the "real" stronghold are shown correctly and I was able to find the previously visited stronghold.

How to reproduce

1. Create a new world in version 1.12.2 using the seed 135996
2. Teleport to the coordinates 1100 70 1500

   /teleport 1100 70 1500

3. Close the world, and reopen it in the latest version/snapshot
4. Use /locate structure to find a stronghold

   /locate structure stronghold

   → There is no stronghold at the reported location

5. Throw eyes of ender to find a stronghold
   → The eyes of ender do not lead to a stronghold