To ensure that we are talking about the same thing, here is an example of what as segment using 999 walls looks like. In this case I also changed the infill pattern - which will never show up while using infinite wall - so that one doesn’t confuse infill with walls. As you can see, these are solid lines. This particular model was a desk clamp I made that had to stand up to the same rigors a metal clamp would have. When made from either PETG or PC, it was nearly as strong as steel. You can see there are zero gaps.
If you’re using 999 walls and not getting these results then there is some other setting that is interfering.