Hum, strange. Just tested it here again, and working fine both with mini-mediacard and by using services. Here’s my card setup
cards:
- type: vertical-stack
cards:
- type: custom:mini-media-player
entity: media_player.heos5_office
artwork: cover
source: icon
toggle_power: false
hide:
power_state: false
icon: true
speaker_group:
platform: heos
show_group_count: true
entities:
- entity_id: media_player.heos5_office
name: HEOS5 Office
- entity_id: media_player.heos1_kitchen
name: HEOS1 Kitchen
- entity_id: media_player.heos1_flex
name: HEOS1 Flex
- entity_id: media_player.heos_stue
name: HEOS LivingRoom
And here is how it looks (notice the +1 on the name after I grouped two of the speakers):
Does your setup look somehow similar?