Strange marks on background of image post editing

Hi,

I am trying to edit out a plain background that had lots of marks/shadows to a clean, plain background.

I use this method:

Edit lighting, contrast etc

Touch up photo with clone/patch tool

Duplicate layer

Surface blur > create layer mask > invert

Fill in the background with the surface blur

Go back to the layer (not the layer mask with blur)

Select the exact colour I want, use a soft brush tool with opacity of around 50%

Go over image with brush tool.

The image looks exactly how I want it to.

I then flatten (have also tried not flattening) and export as jpg.

However, then once the image is saved and I view out of photoshop, the background has these strange marks all over it. They can look slightly like oil spill marks , or just lighter blobs.

It does appear to be where I have edited the background, but I don't understand why when I am in photoshop the image looks perfect but then out of it looks awful!

Please help