Px4flow wrong zero bias/value when the quad is just sitting

Greetings. I have a problem with my px4flow camera, attached to a quadcopter running an ArduCopter.

Per the setup instruction, log reading of body and flow must coincide, but when I tried putting the quad indoor, outdoor, and about 5 points in between, the indoor values don’t coincide

I think it’s supposed to be zero, though maybe a bit noisy. The zero bias varied between those points (negative X and positive Y with varying maximum, the worst at 2 on the graph), with exact coincidence achieved outside, under the sunlight.

I looked for similar problems everywhere, but no-one mentioned about wrong zero bias. I don’t have any idea either about what might have caused it. I have turned off all lights that might have any flicker, but the problem persists.

Any help would be appreciated.