The selection logic takes only the stated width of the stroke into account, not the visual width when altered by the pressure curve. This means that if you draw strokes with significant pressure sensitivity (e.g. for dynamic linework with pencil or vector brush tool) you will not be able to select visually distinct strokes by clicking on them. You instead have to use the alt-key to select behind, or use the layer panel.
Tested on Windows as well as iPad. Likely universal across platforms and Affinity products.
220531 selection bug.afdesign