Spider Spawner changing spawning entity to pig

What I expected to happen was...:
Place a spider spawner that stayed as a spider spawner

What actually happened was...:
The spawner will spawn 1 -2 spiders, then change to a pig spawner

Steps to Reproduce:

1. use command:

/give @p minecraft:spawner{BlockEntityTag:{SpawnData:

{id:"spider"}

}} 1

2. place spawner in light level >7