Random Photos in LR Mobile iOS with distorted background compared to LR Classic counterpart?

I checked your LrC Develop settings to see if something was over-the-top. You have the Sharpening Amount set to 115, which is causing the Smart Preview artifacting. I changed it to 50 and the image is still very sharp with no artifacts in the Smart Preview. Forgot to check LrC Export module Resize 2569 Long Edge. The artifacts are still there so we really need to see the original CR3 file with your settings exported to DNG.

I downloaded th DNG file and exported it in LrC to Lossy DNG with 2560 long edge, which is the same as Smart Previews synced to the Cloud. I can see the same artifacts. However, the DNG file does not appear to be created using the original CR3 raw file. (see below) The LrC Basic panel Profile only shows Color or Monochrome profiles indicating the DNG file was created using a non raw file (TIFF or JPEG). We really need to see the original CR3 raw file with your settings applied that exhibit the artifacts when synced to the Cloud. Also please use the LrC Export module to create the DNG file from the CR3 file using the settings shown in my reply above from June 1.

History Parameters : converted from image/x-canon-cr3 to image/dng, saved to new location*converted from image/dng to image/tiff*converted from image/tiff to application/vnd.adobe.photoshop*from application/vnd.adobe.photoshop to image/vnd.adobe.photoshop*converted from image/vnd.adobe.photoshop to image/dng, saved to new location

"Yes I see them on the web at the site you provided."

lightroom.adobe.com is a universal link that takes the user to their uploaded image files in the Lightroom Web app. As Jao mentioned LrC synced collections upload resized DNG Smart Previews that use lossy compression. However, the compression artifacting in the image files you posted is unusually high. This could be due to one or more of the settngs you applied in LrC. To determine that it would be helpful if you could export the flower image to DNG file format with the below settings. Make sure the Develop settings you used that exhibit the issue are still applied as they will be embedded in the DNG export file and allow us to see what's going on.

I sync a few folders from LR Classic to LR Mobile iOS. Occasionally a photo in mobile will have some odd problem with the background (not the whole photo) being pixelated or washed out but the actual photo in LR Classic does not appear this way.

 

By @jmorg65

Look at the photo on the Web at lightroom.adobe.com. Do you see the same artifacts there?

This is a limitation of the way Classic syncs to the cloud. It only uploads smart previews which are downscaled dng copies using lossy compression. On some images such as what you show here this can cause posterization artifacts that are just due to the compression used in the smart previews. Classic unfortunately and very annoyingly cannot upload full raw images to the cloud but that is the solution to this problem. This can be done through Lightroom Desktop which is the port of the mobile Lightroom app to desktop operating systems. When you open that you will see the cloud synced images in there just like on mobile. Just drag the raw files you need onto it and it will upload them and place them in the cloud alongside the smart previews. This should allow the cloud to create better image previews and export higher quality images.

Add your voice here if you also think Classic should be able to sync full raw images: https://feedback.photoshop.com/conversations/lightroom-classic/lightroom-classic-should-be-able-to-sync-full-raw-files-to-the-cloud-not-just-smart-previews/5f5f45df4b561a3d426443a6  

There is no foreground or background in photos, at least no from a technical point of view. What you see is probably some kind of clipping that probably stronger visible in the background because of the less contrasted colours. It may well be that it is just visible on HDR screens.