Stabilizer gives different results on optimized media

When I was exporting my timeline yesterday, I realised some of the stabilized shots weren't so stable anymore compared to the result in my timeline. After some troubleshooting, I realised that the Optimized Media had something to do with it.

When I delete the Optimized Media, which is set to half resolution DNxHR HQ, and I track the shot again with the same settings, but this time Davinci uses the source files as a reference, I get a far more unstable shot, but the same as an exported version. Tweaking the settings doesn't help. The result on Optimized Media is far better.

I do not understand why. Can you help me?

I'm running Resolve 14.3 free on a Windows 10 system, Timeline resolution has been set to 2048x870.
Source files where the problem occurs are shot on Arri Alexa Mini 3840x2160 Apple ProRes 4444. Most of the (not stabilized) source files are shot on 2048x1152 ProRes 4444

I'm thinking of optimizing the shots that have to be stabilized and set the render settings to 'use optimized media' as a temporary solution.

Okay, so after one big headache for 14 days, I had kind of an "eureka" moment.

After some trial and error, I realised that the stabilizer actually made the shots I was previously talking about even worse. On translation, especially the Y movement became much worse. It looked like it stabilized in the wrong direction. But on Optimized Media, despite the resolution and whatsoever, the result was what it needed to be.

Why? This is where the eureka comes in..

The source files of the first day on which the problem occurs, were internally swapped in camera (Alexa Mini), vertically and horizontally, because the camera was mounted upside down in the rig we used. After transcoding the files using Optimized Media, the problem disappeared. The next day, we forgot the swap internally, and those shots, practically the same, did not result in any problems when stabilized.

So I think this has something to do with the fact that we swapped the footage in camera. X and Y is swapped, the stabilizer seems to be correcting in the wrong direction, on transcoded media the problem disappears.. 1+1=2 right?

I'm willing to share the source files with Black Magic for further investigation if needed.

If you are having the same problem as I do, just transcode the footage. You can use Optimized Media, but make sure to use a good enough codec and resolution, check the "use optimized media" box when exporting the project, and don't forget to wipe existing optimized media of other files where Davinci should use the source.