I tried out your method. In detail, changed the decimal places to a higher value (the highest is 6), changed the document units to pixels and adjusted the curves in the transform panel (some were now 299,998 pixels wide and changed them to integer numbers, e.g. 300), and also restarted affinity.
The issue persists and can be recreated: New document, draw any Bézier curve (probably any shape works, too), add white outline, export as PDF. The outlined curved gets rasterized in that process and the pixelated curve has some black rim at various zoom levels. Apparently the rasterization of "unsupported properties" (fx are all unsupported, right?) plus oddities in PDF viewer messes things up. I guess I have to use another method to add a white outline to shapes. I think creating a copy of a colored curve, coloring it white, add larger line width and placing it behind the colored curve should be the same as outlining curves.