I have problems filleting two polylines if fillet lands on polyline control point.
On image there is a red fillet, which i am not able to generate. If changesing radius smaller or larger there is no problem.

Making a fillet that ‘eats’ its entire base geometry (be it curves or surfaces) is always problematic, yes.
You can manually work around this by using the Arc - Tangent command. Pick the vertical-ish line on the left as your first tangent curve and then type in the radius 8. Snap to the other line and pick the inner arc.