Strange pixels when exporting transparent images

Hello everyone. I have been trying to find a solution for this issue for several hours, but haven't found anything yet. Maybe it's because I am not sure of how to look for the problem.

I have been trying to export a series of files to pdf. Some of them contain linked .psd files which have transparencies. But whenever I export them, strange pixels appear to the right side of the images, like this:

I have tried several things to fix this, none of which actually worked. I tried merging the layers in Photoshop and even changing the files to .jpg with white background with no results.

I created a new document and inserted the images. They exported fine at large sizes, but when reduced, the same thing would happen.

The only solution I could find was: 1) to reduce the original image size to the size I wanted to export to. And 2) Use the PDF/X-4 default settings. Then the images exported correctly. But when I tried to use the same PDF/X-4 settings but downsampling the images, the problem appeared again.

I am using version InDesign CC (13.0)

Does anyone have any idea of what this could be? Thanks a lot in advance.