Select Subject creates unwantedy thin masks on edges of other subjects

Nothing wrong. You are combining individual selections based on antialiased masks. In the computer world, 0.5 + 0.5 isn't always 1 in terms of transparency and that's why you get the artifacts with the partially selected pixels as already explained.

Mylenium

Test like this: use lasso to create a selection similar to what you have. Create a freeform selection using any tool but do not use a selection method converted from a continuous tone image.
Apply Stroke, if extra lines appears then try with preference reset https://helpx.adobe.com/photoshop/using/preferences.html

If no extra lines appears then everything is fine, when you are creating a selection from an image where some pixels are not fully selected thus no marching ants appears. When you use Stroke it fills "non visible" selection which appears as not fully opaque lines.