Is this expected to happen with different versions of drivers?
Yes, the OptiX AI denoiser lives in the driver since OptiX 6.5.0.
If so is the difference between drivers caused by different default model passed to the optixDenoiserSetModel when data == NULL?
Yes, the AI denoiser can have differently trained networks or algorithms in different drivers with the goal to improve the results and performance.
And if so, is there some way how to store / extract the model used for one particular version of drivers so that I could pass it always to the optixDenoiserSetModel?
No, there is only one network at a time. It’s pretty big already, There actually isn’t even a separate training network for the LDR mode anymore to make it smaller.
And version of drivers where it was clearly possible to see differences in results on Windows were for example: 440.97 DCH vs. 445.75 DCH
Right, the R440 driver branch up to version 442.50 contained a different training network which was improved in that release and later versions. The drivers 442.50 and newer should behave the same so far, but this can change in future versions.