Photo resolution reduced / halved when downloaded from icloud
I'm using an iphone 12 and when my photos are downloaded to the Windows desktop app from the icloud, they all have reduced file size and dimensions. Original photos on my phone are 3024x4032 but when they are downloaded to my PC from icloud they are only 1536x2048. If I email the same photo to myself using 'send as actual size' the photo I get is the correct resolution 3024x4032.
I have 'Most compatible' set in camera format on the iphone. I also have 'download and keep originals' checked in photos on the iphone.
This did not happen with photos from an iphone 6S, they always downloaded at full resolution via the PC app.
Can anyone help please? Is there anything to change in settings to ensure that I get full resolution downloads from icloud?
This is a major problem as I need to be able to easily download full resolution images from my phone and I can't be emailing each and every one of them.
Thanks!
Windows, Windows 6