Created attachment 20027 [details] minimal case for problem Possibly related to 702970 The left square does not show. (also reported at Sumatra under case 3429 and pdfium case 1571)
The Sumatra case is actually a forum post, sorry. https://forum.sumatrapdfreader.org/t/pattern-is-not-rendered-properly/3429 There already has been some discussion of this problem by more knowledgeable people.
Some other PDF viewers I tested this file with also exhibit a variety of problems. Ghostscript does not render the pattern at all on the display device. Using the png16m device it renders a red pattern (solid on low res, visibly cross hatched on 800dpi and up) on both squares. Evince renders the pattern in the stroked square, but not in the filled square on the left. Adobe Reader 9 renders a red cross hatched pattern in both the filled and stroked squares.