Partial layer appears after altering mask in Camera raw

Hi ​@moplatka,

Thanks for the additional screenshots. The latest screenshot helps clarify what is happening.
 

What you’re seeing on the left side is transparency (the checkerboard area), rather than a separate partial layer. Since it appears only after you modify a mask in Camera Raw and return to Photoshop, I’d like to narrow down whether this is related to the Camera Raw mask/rendering process.
 

The fact that this particular image came from an iPhone 15 Pro is useful, but I wouldn’t assume yet that the iPhone file itself is the cause.
 

Could you please try the following:

1. Update to Photoshop 27.10 and Camera Raw 18.6 to the latest available versions and test the same file again.

2. Try the exact same workflow with another iPhone 15 Pro image. Does the transparent area appear again?

3. Try the same workflow with one of your Canon R5 Mark II files that normally works.

4. In Camera Raw, temporarily uncheck all GPU acceleration settings under Preferences (Gear Icon) > Performance, restart Photoshop, and repeat the test.
 

If the problem occurs consistently with iPhone files but not the Canon files, we’ll have a much stronger indication that something about the iPhone file/workflow is triggering it.
 

If it still happens, please also share Help > System Info > Copy from Photoshop, along with the original iPhone file that reproduces the problem (you can send a download link via DM). If we can reproduce it with the same source file and mask workflow, we can take it forward with the product team.
 

Best,

Anshul Saini

Unfortunately, that doesn’t show us anything.  We need the full workspace with layers panel visible.  It needs to be full-res and with the problem layer selected.