[InsainArcaneBirdbrain]

To elaborate what Fern's talking about- normally when a fight takes place with a custom battlefield, it utilizes just the housing tiles and collision exclusively which overrides the grass map. With PvE fights spawned with the battlefield however, you get the house tiles layered OVER the standard grass map, with flowers, bushes, and other objects on top. This only affects PvE, and not PvP.

TLDR: The normal grass battle map is being loaded alongside the custom house one.