So, I have now taken a close look and can confirm that the stereo sound on Bluetooth automatically comes good with standard operation.
Therefore, I fear that your oE400 has a defect and should be replaced under warranty.

We also have these headphones and they are probably the simplest BT headphones. I don't think you're doing anything wrong.
It's interesting that it works with the jack. But not with BT.
I'll have to check how it works with mine ...