Fill tool in grease pencil's fill tool lets a separation with the strokes

Hi blenderartists,
I have a problem with Grease Pencil: when I use the fill tool, it doesn’t fill all the surface, letting a gap between the fill and the stroke. I’ve tried several solutions, even reducing the stroke’s thickness at minimum, filling, and then adding the anterior thickness to the stroke, to avoid that gap.
Here is a sample:


Please, can anyone help me with this?
Thank you!