The bug

When two or more structures with block entities (such as spawners or chests) intersect, it can happen that the data of the block entities becomes corrupted:

• there is a mismatch between block and block entity (can be verified using /data get)
  E.g. the block is a chest, but the block entity is a spawner
• block entity data is reset to default values
  E.g. spawner starts spawning pigs (the default mob)

This also causes errors and warnings similar to the following to appear in the log:

Failed to fetch mob spawner entity at (-1186, 17, -309)
Block entity invalid: minecraft:chest @ fx{x=-1186, y=17, z=-309}

See also videos Pig Spawner in Vanilla Survival 1.16 and Quintuple Pig Spawner Cluster in Vanilla 1.16 and explanation (Discord).

Example seeds and coordinates

Note: Due to world generation changes the following examples might not work anymore. In case no intersecting dungeons generate there anymore please do not claim that this issue is "fixed" if you cannot prove it.

/execute in minecraft:overworld run tp @s -1186.32 18.52 -308.62 -84.90 73.35

/execute in minecraft:overworld run tp @s 435.96 48.37 -117.43 61.20 45.60

/execute in minecraft:overworld run tp @s -1634.34 25.55 -3616.51 143.40 47.10

/execute in minecraft:overworld run tp @s 6105.55 59.00 18970.90 539.25 51.45