Media Player Group - Unexpected Script Result

Need some help. I have 2 Chromecast devices grouped together as a single media_player entity, but for whatever reason I cannot get them both to play at the same time. It’s one other other.

I also made a video (here) to show exactly what’s happening, which is that my test script causes media to switch from one Chromecast to the other.

Here is my config where I grouped them together:

media_player:
  - platform: group
    name: Chromecast Group
    entities:
      - media_player.basement_chromecast
      - media_player.whole_house_chromecast

And here’s my script:

chromecast_test:
  alias: Chromecast Test
  sequence:
    - service: media_player.play_media
      entity_id: media_player.chromecast_group
      data_template:
        media_content_id: spotify:playlist:3kMW0YxxRCg0DAxPGjKm1v
        media_content_type: playlist

Any help would be so appreciated. This is infuriating.