When I use a command to give me a custom mob_spawner, and place manually the block to start the mob generation, it generate one of them, and turn itself into a pig spawner just after.

/give @p mob_spawner 1 0 {BlockEntityTag:{SpawnData:{id:"minecraft:spider"}}}

If I use a setblock command to place the spawner directly, the issue does not occurs. For now i use setblock command as a workaround.

I Saw a lot of issues talking about mob_spawner turned into pig spawner, but not about a spawners doing it right after the first generation.