Do you have a smart trainer or any other source of power that you could verify with? If not, perhaps you have a friend with a smart trainer or power meter pedals that you could use to help you verify the extent of the power offset? Knowing this information would be useful to send Quarq Support if you decide to go that route.
I haven’t personally seen anything like the issue you are describing, but from a little digging I was able to find some information about verifying your Quarq power meter’s slope. This is a somewhat complicated process, however, I’ve included links below that will be good resources for you.
This video helps to explain how the process works:
Cycling Power Labs can be a great tool for helping you with this process.
This thread could be helpful for further troubleshooting:
Like I said, I don’t have personal experience with this issue, but this is just one possible lead that you could investigate. If I were you, I would determine the amount of offsetyour power meter is experiencing, and then send an email to Quarq Support before going down the slope rabbit-hole.
Cheers and good luck!