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.
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.