The sleeve looks enclosed until the paint tool colours part of the background as well. Zooming in reveals the cause: two line ends almost meet at the cuff, but a small opening still connects the sleeve to the surrounding space.
Digital colouring depends on how the application identifies a region. A gap may cause a fill to escape or make the software refuse to fill the area, depending on the tool and its settings. Repeatedly choosing the same colour will not repair that boundary.
A boundary does not always need a visible new line
Toon Boom’s painting lesson describes closing gaps with drawing tools, a dedicated Close Gap tool or an automatic option. Its workflow distinguishes the visible artwork from strokes that can close a painting region without adding another visible contour.
That distinction matters when an open line is intentional. A cheek contour might stop short of the neck because joining it visibly would make the face look heavy. An invisible boundary can preserve the design while separating the colour zones. The exact controls depend on the application and version.
Try a small diagnostic drawing
On a duplicate drawing, make two simple shapes: one completely closed and one with a narrow opening. Fill each using the same settings. Then close only the opening and repeat. This separates a boundary problem from other possibilities, such as painting on the wrong artwork layer.
Return to the character and inspect the actual leak rather than raising every tolerance setting at once. An aggressive automatic setting may bridge an opening that should remain separate. Check the collar, fingers and other closely spaced details after the repair.
Finally, inspect the drawing at its intended viewing size. A clean fill should support the linework without leaving pale slivers or swallowing small details. Record which boundary changed before applying the fix to neighbouring frames; each drawing may have a slightly different opening.