P: (Win) does not sync aspect ratio changes to cloud

Those details are definitely helpful and tell me that what I tested probably doesn't apply.  I'll try to replicate what you're doing when I can get to my camera, although I can't do it with an .arw.

A couple questions - 

 - Is this new behavior since upgrading to LrC 10?

 - Does it only happen with images in portrait orientation?

 - Do you apply any presets on import?

I am using Sony's .arw and .dng files. 

By default my camera is set to apply a 16x9 crop which is respected by Lightroom. After importing my pictures I change the crop for all portrait orientation photos to "Original" which gives me the full uncropped image. Syncing these will lead to a distorted image in all cloud applications. Opening the crop tool in any of those shows me a a cropped frame (16x9) as after import. Thats not correct! And the images are stretched! if I then apply "Original" in the cloud apps the image will be displayed correctly there.

Seems to be working fine for me with LrC 10.0 & Win10.  I tested with dng, heic, jpg, & cr2 files that started both in LrC and also ones taken with the Lr Mobile (iOS) camera app.  In all cases, changing aspect ratios & cropping synced properly back and forth between the all platforms - LrC, Mobile & Web.

A step by step and/or video of what you're doing & seeing might help identify what's going on.