Hello,
Some additional information on why this behavior happens can be referenced here:
Generally your usage of the Update overload Update(Annot, int) would redraw only the given area.
Depending on the annotation or load required PDFView may need to do a full update. This behavior can be seen if you draw multiple annotations in quick succession with our demo as well. A direct solution to what you’re seeing here would likely involve toggling SetProgressiveRendering as this would allow you to more control when the flickering needs to happen.
Please let us know how this works for you.