If you wish to reset the files which you have already imported, please follow the below steps.
> Go to Library module and choose Grid view
> Select all the images which you want to reset
> Make a right click or CTRL + Click on any of the image
> Then Choose Develop Settings > Reset
For future imports, please make sure you have selected None in Develop settings in Apply during Import in the right hand side of the import dialog box.
Looks like it worked but it came back. I don't have any develop settings upon import in the first place so not sure where this is coming from!
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Is it possible you set the default camera profile with a vignette applied?