The bug

Dungeons in strongholds sometimes generate incorrectly with the stronghold overlapping the dungeon, resulting with the spawner not generating. When the dungeon spawns in a chunk, while extending out to a different chunk, the stronghold will overlap the dungeon in the chunks that spawn outside the initial spawn chunk (not the spawner, the part where the dungeon decides to spawn, the wall).

How to reproduce

1. Use the seed -1002375371929129041 with the default world type
2. /execute in minecraft:overworld run tp @s 8116.50 24.00 1457.50
   → The spawner in the dungeon does not spawn due to the stronghold generating after the dungeon when it passes it's spawn chunk.

Notes

You can't reproduce this in 1.15.2 or earlier.

Relates to MC-125006 (doesn't specify the spawner not generating)