Phone 13 Bluetooth not stereo

At home I stream music from my iPhone 6s to an older stereo receiver using an Esinkin Bluetooth adapter. This works perfectly. I recently upgraded to an iPhone 13. The new phone connects to the adapter, but I only get output from one speaker. That’s one channel, no stereo. Both phones are using the same IOS version and music from my old 6s is still fine. So… am I dealing with an incompatibility between my iPhone 13 and the Esinkin BT adapter or is there a setting on the iPhone that is the culprit? Any help appreciated.