Volume control of Spotify Connect device fails on iOS device

[ChonkyCat]

Plan

Premium

Device

• iPhone 14 Pro
• Spotify Connect device : Harman Kardon Multibeam 1100 - software ver 23.09.51.81.01
  

Operating System

iOS 16.5.1c

Spotify app 

8.8.54.544

Volume control of Spotify device (soundbar) via physical volume buttons on iPhone or volume slider in Spotify app would work fine for a while.  After that, the volume control on iPhone would just stopped working.   It could be days/weeks before the soundbar decided to give up and refuse to listen volume control commands from my phone anymore.   I have to power off (mains off) to reboot the soundbar to get the volume control working again.  I do not know what triggers the failure nor I can recover the device from failure except power cycling the device.   Killing the app does not help because the same problem happens to multiple iOS devices in my household under different spotify accounts, so I am pretty sure it’s either the app or the Spotify Connect device misbehaving.   
Reading from the long pages of comments and feedback in other similar posts, seems that this is a recurring issue and either Spotify is not interested to fix or simply clueless to fix ?

[s08e80m8]

Using a Yamaha RN303 Receiver. I can control the volume from my Macbook pro via the native app (Spotify for macOS (Apple Silicon) 1.2.17.834.g26ee1129) but not from iOS (v8.8.58.450).

Receiver is hard-wired to AP via unmanaged switch.

Macbook & iphone both connected to AP via WiFi