Join currently takes the sound and vibration configuration from the source app to mirror it to the destination devices.
The problem arises when someone works with multiple devices. People may not want their tablets to vibrate when a Join notification is received, but just a sound, and the other way round with their Android smartwatches (only vibration, even when the source device is configured with no vibration).
For this reason, I propose a new feature to override notifications behavior in each receiving device as an extra option. To make it easy, I suggest starting with the implementation of a 'general default behavior' option for all incoming notifications, instead of per-app override, which could come later.
This feature would also help to support some apps that are not so well-known and don't work so well, f.e. Disa, where Join just pushes the notification without sound or vibration despite the actual app configuration.