Just to put a bow on this, the root of the issue was that the speakers were on a WiFi network type of IoT. This will restrict intra-LAN communication, particularly sourced from the IoT clients.
It is common, when grouping smart speakers, one speaker is selected as the coordinator of the group and, in doing so, will need to communicate to the other speakers to perform the coordination. The IoT network type blocks this communication as a security measure given the questionable security of some IoT devices.
The fix was simple enough, put the smart speakers on a different network type. In this case, Standard/Small was used and that resolved the issue.