LightBurn is doing exactly what you asked it to do with these settings. Yes, changing the font can have this result if the font is not the same size as you have not adjusted the gap to accommodate this “taller” font. Look at this example:
2 rectangles (black and blue), each set to ‘Fill’ but with different intervals. Red layer has a single line as a reference to the bottom of each rectangle to show where the Fill ends. Notice that the blue rectangle fill comes up short of the bottom. This is due to the gap between each fill line, and the next line calculated to be drawn in the fill would fall outside the rectangle, thus not drawn.
As for your other attached image, I am not sure what you are showing, but the cut looks like there was some bouncing happening during this cut. Is this what you are trying to show?