sen0321 clips at 20ppb

I have had the sen0321 ozone sensor for 5 days. It clips the
output from below at 20pbb, meaning that all readings that
should have been below 20 are published as 20. From a graph
showing quite a lot of fluctuation, it is clear that when it
gets down to 20 it is just constant, with no fluctuation at all.
(The graph really looks like clipped audio up-side down.)

No analogue signal could possibly produce this result, so it
looks like a problem with the analog-to-digital conversion.

(I run the unit using esphome. I looked into the code for this
implementation. It is very simple and does not look like it
could be responsible for the clipping. (The code also looks a
lot like the python script provided by DFRobot.))

I am coming to the conclusion that my unit is defect, but I
would be very happy if anybody could comment on the issue,

or even help find a solution.