So in a demo I set up a simple image of two concentric circles to show an offset fill in action.
Cut settings as in the screenshot
project attached
odd_fill.lbrn2 (7.7 KB)
There is a lot of math going on inside LB to work out the best path for your offset fill.
It is one of my favorite tools to use, although many on the forum are not fans of the setting.
Your two circles that you are trying to fill between are just far enough apart to fall prey to the rounding errors of the maths. Try adjusting the interval just a little bit, and you will see a different patter emerge from the rounding errors. On a more angular shape, you will get a patch work quilt effect as the math breaks down your shapes into smaller increments. A square will be broken into 4 triangle shapes, meeting in the middle.
Note - some shape will cause the maths to get rabies and go mad. The 4-H four leaf clover design will not work with offset fill. I have learned the hard way on that one.
Play around with interval and object shapes to better visualize the process. FWIW, offset starts in the middle of the shape and build concentric layers outward, like a pearl adding layers. I like to use it for large text, knowing that I may have some small parts that get skipped on the inside of the letters. It’s part of the game. Oftentimes, the run time for offset is considerable shorter than for the other fill methods. You are on the right path by using the PREVIEW button.
Keep up the exploration and keep having fun 
The distance between the two circles must be a multiple (plus a little bit more) of the line spacing to get a solid fill. If it’s slightly less than a multiple, then Offset Fill must leave a gap that can be up to the line spacing.
The 0.5 mm line spacing shows 7 lines fit on both sides of the average radius of those circles, plus one more line where the rounding goes the other way. That means the two circle radii differ by just slightly less than 2×7×0.5 + 0.5 mm, which means they’re about 7.5 mm different.
Guessing: circle diameters of 60 and 45 mm?
Which is why Offset Fill is generally a Bad Idea.
Thanks for the explanation, I had assumed that it would start from one circle or the other, not both.
Some shapes you will see a direction change during offset fill. I don’t know why, but I have seen it.
Offset Fill is not the same as Fill, and a couple of times I have seen Fill take less time on the same project. I always check both, and sometimes choose Offset just for the unique burn patterns.
