I've just seen a problem while running the fill command:

[PaperChunkGen-world/ERROR]: Failed to fetch mob spawner entity at (-7233, 29, -2073)

I went to this location and the spawner was a pig spawner...

I only have worldborder installed on this server so it cannot be caused by another plugin.
I'm using WorldBorder 1.19.0 on paperSpigot 1.13.2 (473).

Edit: I've fixed them manually and when went to them:

[Server thread/ERROR]: Block at -6 497,46,286 is minecraft:spawner but has net.minecraft.server.v1_13_R2.TileEntityChest@4c197bd3. Bukkit will attempt to fix this, but there may be additional damage that we cannot recover.

So it's probably not a problem from WorldBorder but in the world generation itself. As I understand, the dungeon chest was generated in the same block than the spawner which causes this bug.

Sorry about this reporting if it's the case but I prefer to report just in case.

Yeah, that would definitely be down to Paper / Spigot / Bukkit / NMS code if you have no other plugins or mods.

Thanks for your quick confirmation, so you can close this issue I think.
Sorry to have opened it, I though it was related to WorldBorder when I did.