Photoshop: Rectangle appears on images after editing in Photoshop and returning to Lightroom Classic

If you disable GPU in Preferences > Performance and restart Lightroom do you still see the artifact?

Still there.

If you Export to a JPG and then view that JPEG file in Mac Preview is it still there?

Can you provide a copy of the file? 

If you open the file in Photoshop, go to Preferences > Performance... and click "legacy compositing" and resave the file, does it appear correctly in Lightroom?

Yes, it is still there even after export, this is why this is so bad, I can't do any work.

Still the same if I put on Legacy, edit a layer and save, same rectangle in Lightroom.

The file 350MB

Iain, I just posted this reply to your original thread in the user-to-user forum:
https://community.adobe.com/t5/lightroom-classic/square-image-in-my-images-after-importing/m-p/11000...

"Reading through the jumbled order of replies, it's not clear that you've tried enabling the Photoshop option Preferences > Performance > Legacy Compositing, as suggested by Bob Somrak, and then resaving the image after making a trivial change. There is a bug in PS 21.0 and 21.1 that incorrectly renders the final image as seen by LR and other apps (PS displays the image correctly):

https://feedback.photoshop.com/photoshop_family/topics/reproducible-bug-in-feathered-layer-masks-photoshop-lr-classic?topic-reply-list%5Bsettings%5D%5Bfilter_by%5D=all"

I have done that 3 times now, it makes no difference.

I don't think this is the same bug, this is an original Cr2 file in Lightroom, basic edited in Lightroom, opened to edit in Photoshop, saved and closed and now this is what it looks like in Lightroom.  Therefore it's a Tiff, not a PSD.

I think the most effective next step is, as Jeffrey Tranberry suggested, to upload a sample file showing the problem to Dropbox or similar and posting the sharing link here. That will help identify whether the problem is specific to the file or your particular configuration.

Ok, here is the file.  I closed and opened Lightroom 22 times and now its gone. Surely I can't be expected to do that in my workflow.....

https://www.dropbox.com/s/5v5st04ayy5t0jm/6N8A6210-Edit-Edit.tif?dl=0

I just retested, and the bug with feathered layer masks occurs with both PSDs and TIFFs.

I'll be doing some editing tonight, I will post an example when there is no feathered masking, I'll upload the Tiff etc.

Thanks!

Let's tease apart concerns and focus on the image(s) with feathered layer masks first.

Re your sample TIFF, I don't see the symptoms when I open it in Mac Preview or LR. But some further testing shows that the bug only appears to occur when you open a raw in PS, add a feathered layer, and save it as a PSD or TIFF. If you reopen that PSD or TIFF in PS and force a resave, either by making a small change or by doing File > Save As, the image then appears correctly in LR and other external apps.

So it's the first time you open a raw and save as a PSD/TIFF that provokes the bug.  Subsequent PS edits to that PSD/TIFF cause the bug to go away.

(Note that I've eliminated LR from the equation -- all my testing is with Photoshop and Mac Preview, which reads the compatibility layer to display a layered PSD or TIFF.   The bug is rendering the compatibility layer incorrectly.)