Bose SoundLink around-ear wireless headphones II only works on 1 side when using bluetooth but when the cable is plugged in, both speakers work just fine. Also, this is the case on multiple devices and I've verified the firmware is up to date.
Thank you for posting! I appreciate the detail you've provided, that's very helpful. Because the issue is consistent across several devices I'm thinking that it could be defective. I'd like to try a reset of the headphones:
Turn the headset off and wait for 30 seconds.
Plug the headset into a USB power supply via USB cord and wait 5 seconds. (Power supply needs to be plugged into the wall with power on).
Unplug USB cord from headset and wait 1 minute.
Turn on the headset and test if the headset works properly.
Afte you've completed the reset let's clear the pairing list on the headphones. To clear the list you'll want to slide the power switch all the way to the top towards the Bluetooth symbol and hold it there until you hear a voice prompt notifying you that it is cleared, this should take about 10 seconds. At this point make sure to forget the headphones and delete them from your device's paired lists, then you can reestablish the connection.
If this doesn't help then it would seem that there is an issue with the headphones and I would suggest reaching out to customer support here.