When exporting to a vector format (e.g. PDF or SVG) or when a line subsymbol has dynamic (data defined) properties, we now automatically switch to a line-by-line based approach to render the fill. (Previously, a tiled pattern approach was always used, which often resulted in undesirable artefacts and visible seams in the pattern).
Some of the many benefits of this change include:

• Smaller PDF/SVG output file sizes -- the line pattern fills aren't stored as raster images in the output so the overall file size will often be much smaller
• PDF/SVG exported files are easier to modify in external applications (such as Inkscape or Adobe Illustrator) for post production, as each individual line in the pattern can be modified.
• Better quality PDF/SVG outputs, since the fill isn't DPI dependent and looks pixel perfect regardless of how far in you zoom
• No visible artifacts at certain angles, distances or with certain line symbol styles

And even more excitingly, it opens the door for a range of new symbol styles, for example:

• line patterns where the individual lines change color/width/dash/ etc.
• line patterns with marker line symbols on center point/etc
• geometry generator effects per line, e.g. wavy line patterns, hand drawn line styles, etc