Artefact errors with selection: is this a bug, or my fault?

@Mllejules Hi, I hope I've understood correctly, it would be useful to have close-ups of an area of your image pre and post blur so we can see the artifacts clearly

As this a Jpeg image, those could easily be Jpeg artifacts - since you are blowing it up massively on-screen (568%), those are kinda inevitable. 

Just as a test please try a whole number view, say 500% rather than 568% which will give you a more accurate view of your image content (but I doubt it will disappear the artifacts). 

It could be that when you take this image back down to 100%, the artifacts you have will not be visible. I hope you're not planning to use it above 100% as that's when artifacts start to show themselves - even in some unmodified Jpegs.

A few notes on Jpeg:

Jpeg files have compression applied, changing resolution or cropping and re-saving enhances the compression artefacts - this means that Jpeg is only really suitable for final file delivery/transfer - with the Jpeg created only once size and resolution (and any sharpening) have been completed. 

So,Jpeg is a far from ideal for editing and not OK for archiving or for any file that may need to be resaved, resized or cropped down the line.

Jpeg is the worst possible format if you want to keep high quality - you should always archive a copy of your original, ideally with any adjustment layers intact - if you work with layers.

Jpeg compression (at any setting*) really is "lossy”, irreversible and cumulative, so Jpegs should ONLY be used only for final delivery and only created from the original file format AFTER resizing & cropping to the FINAL size and crop.

To explain: any edits to size or crop, or even just re-saving a Jpeg file means further compression, potentially that’s very damaging.

The JPEG compression damage is not always immediately apparent, which is perhaps why it's still widely used - however, that compression will soon cause issues if you do further work and save again. That’s when you’ll see a Jpeg with some real issues.

*don’t imagine that selecting maximum quality for your Jpeg is preserving the original data, it’s still compressing a lot which discards information.

SO, don’t reuse Jpegs if any resizing or resaving is needed. Always go back to the PSD/ Tiff originals, Jpegs are essentially a 'use once and trash' file type.

I hope this helps, sorry if it's disappointing news
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management