Sprites inside an sprite atlas are included in the build separately

We're using the new SpriteAtlas system to generate atlases for the frames of the animations of all characters. I checked the Editor logs and it seems that every single frame of the animation is also included in the build. I guess this is because these frames are referenced in the animations. I never had a problem like this before using the old sprite atlas packer.

Is this a bug or am I doing something wrong? Is there any workaround to fix this?

 

Update: did more digging, and it turns out that Unity keeps very small files with the same name as the original sprites in the build like this:

Spoiler: Editor Log

6.7 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/13 Abortion/Body/26.png
6.7 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/08 DriveLeft/Body/09.png
6.7 kb 0.0% Assets/Content/Characters/Pregnant/Animations/Main/FlyToGoingDown.anim
6.7 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/04.png
6.7 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/02 Detection/Body/10.png
6.7 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/02 Detection/Body/00.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Animations/Main/StunStart.anim
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/12.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/02 Detection/Body/02.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/10.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/06.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/03 LiftOff/Phase3/Body/00.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/11 StunStart/Body/02.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/14.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/00.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/08.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/22.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/20.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/18.png
6.6 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/16.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/22.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/20.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/18.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/16.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/14.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/12.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/10.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/08.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/06.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/04.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/02.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body2/00.png
6.5 kb 0.0% Assets/Content/Characters/Pregnant/Frames/Main/01 Idle/Body1/02.png

The files are a lot smaller than the original files, so I'm guessing that these files keep some info about the original sprites to make sure that the animations won't break. But it would be great to have more info from someone who worked on this. Is there any documentation for this behavior?