drawn circle not filling in properly, paint bucket not working properly

That is completely normal and standard...

And there is no easy solution to this. Well at least not in a general manner. For your example, you can either

• increase the tolerance
• select the interior and let grow the selection
• use a different method to draw the outer lines (like shapes)

Create any crisp Shape with the Freeform Pen Tool and settings in its Options bar.

Let's see whether it might caused by the brush stroke or the Paint Bucket. Draw the random shape with a hard edged brush. Then, Magic Wand inside it (zero tolerance)  and  Edit > Fill with the same color.

still does it with a hard brush, also adobe Photoshop CC 2017

This normal since the brush is a soft brush. Try the harder brusher without soft edges.

what program?