
Welcome to InkscapeForum!
Well, twice I've had a reply ready to post, but when I post it, the message is gone! (Not your fault, it's part of our spam prevention -- when you edit the msg, it has to be approved again.)
When the sketch effect is applied, it looks like the original closed path is exchanged for the several open paths which comprise the effect. Paths which are not closed can't be filled completely. Any concave areas will accept the fill, or the other side of convex areas. But to have a complete fill, you need a closed path.
One good solution in this case, might be to duplicate the object and apply the effect to the duplicate. Leave the original close path filled, but remove the fill from the duplicate after you apply the effect.
There may well be other solutions to this, but that's what I would try 1st
