icloud photos under Windows have half resolution since moving to iPhone 12 Pro

When I moved from iPhone XS to iPhone 12 Pro end 2020, I noticed that the resolution of my photos synced via icloud for Windows is only half of the original resolution. On iPhone they are 4032x3024 (DCIM Folder in Windows Explorer) but the same photo is only 2048x1536 when synced to PC.

The same effect happens when I use a browser to download from icloud.com. The photo is only half of the resolution in both directions.

BUT: when I use a different PC (e.g. MAC from Apple support person in the apple store), the photo downloads in the right and original resolution! The resolution is also correct in my Windows PC when I sync Portrait-Mode photos.

So this must be an issue with iTunes/Windows/iPhone12 combination because other phones or other PCs/Laptops do not show this behaviour.

I went through many blogs, support topics, internet searches but didn't find a solution. As the problem didn't happen at Apple support desk, they also couldn't help. There are a couple of people with the same problem, but none of the discussions ended with a solution.