What I see in video is that the OSD horizon lags very far behind the true value. That might be caused by any of several different things.
The lag is the problem I see. The next question is whether that lag is only for the OSD or causes bad flight problems too.
The first thing I would do is to connect a USB cable to GCS and do the same test while watching the GCS "Flight Data" page which has a similar "artificial horizon". It should respond immediately! Note that it will tip in the opposite direction you tip the quad. That is normal (think about it
).
From your recollection, does it fly in Attitude mode and automatically level "quickly" when you release the sticks? If so then it is just an OSD problem (probably needs higher baud rate for both sides of OSD connection).
If it does not fly in Attitude mode (does not level correctly, the true leveling has too much lag) then I ask whether you used a "cloud config" or whether you adjusted some expert values like GCS -> Configuration -> Attitude -> Settings -> Filtering (or) GCS -> Configuration -> Stabilization -> Expert -> Sensor Tuning. Reset these to default values to make sure.