Here's the output from the broken PC, it seems to work on another PC so I really don't know what I'm doing wrong.
{"availability_topic":"homeassistant/sensor/THEATER-PC2/availability","command_topic":"homeassistant/button/THEATER-PC2/mediaplaypause/set","action_topic":"homeassistant/button/THEATER-PC2/mediaplaypause/action","unique_id":"c9fcb6d4-aad8-4454-a990-4861290e64e2","object_id":"THEATER-PC2_mediaplaypause","device":{"identifiers":"hass.agent-THEATER-PC2","manufacturer":"HASS.Agent Team","model":"Microsoft Windows NT 10.0.26100.0","name":"THEATER-PC2","sw_version":"2.1.1"},"entityname":"mediaplaypause","name":"mediaplaypause","state_topic":"homeassistant/button/THEATER-PC2/mediaplaypause/state"}
Here's the working one
{"availability_topic":"homeassistant/sensor/PHOENIXLIBRE/availability","command_topic":"homeassistant/button/PHOENIXLIBRE/PHOENIXLIBRE_mediaplaypause/set","action_topic":"homeassistant/button/PHOENIXLIBRE/PHOENIXLIBRE_mediaplaypause/action","unique_id":"6d8e40b9-71d6-4d16-966f-0d8ebbbb201e","device":{"identifiers":"hass.agent-PHOENIXLIBRE","manufacturer":"LAB02 Research","model":"Microsoft Windows NT 10.0.22631.0","name":"PHOENIXLIBRE","sw_version":"2022.14.0"},"name":"PHOENIXLIBRE_mediaplaypause","state_topic":"homeassistant/button/PHOENIXLIBRE/PHOENIXLIBRE_mediaplaypause/state"}