Just to expand/rephrase the answer @billy_kerr gave:
When you click the
paintbucket tool inside an area, it has no idea that the cloud you drew with the
paintbrush tool is a line. Everything in your image is just tiny squares (pixels) and when it spreads out and meets each pixel it has to decide if it's something it should fill in or avoid, ( basically whether it's the same colour as the pixel you originally clicked on or not).
If the cloud line you'd drawn was pure black and each pixel was either pure black to signify the line, or white, to signify it's not part of the line, the
paintbucket tool would have a pretty easy job of deciding what to fill in. However, even the blackest of lines drawn with the
paintbrush tool in PhotoShop do not have pure black pixels at the edges of the line. They're various shades of your chosen colour to simulate soft edges, rather than harsh jaggy (aliased) edges. You're probably familiar with how this looks from zooming in.
So when the
paintbucket tool gets to your
paintbrush line that you drew for your cloud, its not meeting one colour. It's meeting lots of various shades of that one colour and needs to decide if it should fill them in with the
paintbucket colour or not.
Still seems easy, but if you go to the farthest edges of your cloud line you'll see some of the pixels are closer in colour to what was behind them (possibly white background) and some closer to the line colour. What does it do there? That's where you get the problem you experienced, where
paintbrush decided the edges of your cloud line were probably something that wasn't meant to be filled in and abandoned filling at the point where you see the faint transparent edge.
You can adjust how close a colour has to be to what
paintbucket is trying to fill in by highering the tolerance option. The higher the number, the more
paintbucket will treat deviations from the original colour you clicked as something it should probably fill in.
And, as already also mentioned, this has nothing to do with using a Wacom, it's just how the
paintbucket tool works and is a bit of a hangover from the early days of digital images.
Most illustrators keep line work done with the
paintbrush tool and fills on separate layers if they're two distinct renderings. The line work is the upper layer and you can colour under it on the fill layer, aiming to underlap the fill slightly so that the lines hide the imperfections of the fill boundary (often done with the
paintbrush tool also - similar to a broad marker - instead of the