You need to add an entity list inside the sonos_master_state. Every entity_id that pops up as a state for states(‘input_text.sonos_group_master’) needs to be in that list otherwise it will not work. Templates cannot create listeners if it doesn’t know what to listen to and that is the root cause of the problem. This is the same issue that now() has. No entity_id, no updates.
sonos_master_state:
friendly_name: Sonos Master state
entity_id:
- media_player.one
- media_player.two
- media_player.three
value_template: >
{{ states(states('input_text.sonos_group_master')) }}
Templates have been like this since the begining of HA. It’s why the entity_id attribute exists.