If a block in the #features_cannot_replace tag is already occupying the central spot in a dungeon, the dungeon spawner will fail to generate and an error (Failed to fetch mob spawner entity at (x, y, z)) will be logged. Furthermore, because the dungeon chests are generated before the spawner, it is possible that one of them will be placed in the middle of the dungeon and act as the spawner inhibitor; the placement algorithm, which can place a chest in any location adjacent to a solid block, makes this possible if another structure (such as a mineshaft, stronghold or even another dungeon) is providing additional unreplaceable blocks. There are several ways to fix or minimize this bug:

1. Firstly, dungeons should not be able to intersect other structures at all. (MC-186343, MC-125006, MC-211255, MC-221960, etc).
2. Then, if this cannot be helped, the spawner should generate before the chests so that its placement only fails if the intersecting structure itself is obstructing it.
3. If it is still decided for whatever reason that the chests should generate first, then their placement should fail outright or be moved to a different location if they would be placed in the middle of the dungeon.
4. Failing all that, the console message should be changed to a warning and this issue should be resolved as fixed or intended.

Below is an example of the bug. Create a new world with seed -3641748469545596142, go into spectator mode and run /teleport @s 245 -2 -276.

See also this post on the Mojira Discord for context.