Is it possible this is not a device bug but rather an OS quirk? If you are in an app using audio and press the volume buttons they control the volume for that app type. If you press the volume keys while nothing is running it adjusts the ring volume (at least it seems to on my oneplus one). Now for the real quirk, the buttons still adjust the volume while the phone is asleep. To test this turn the volume all the way down, press the power button to put it to sleep then press the volume up ten times followed by the power button. You will see the on-screen volume level and it will be up. If it is not on-screen press the volume button one more time to bring it up and see if the volume was adjusted while the phone was asleep. So it may be the volume is being turned down by the buttons while in your pocket.
I have updated my E5 to UBPorts using the magic device tool and all went fine. The os is good on my phone and I like it so much I have moved my M10 over too. I have moved a friends OnePlus One and this works good too. I wooed suggest you go for it.