Strange stroke behaviour on transparent objects in PDF

i'm trying export poster designed in illustrator cs4 pdf file. there several elements semi transparent , overlapped each other.

 

objects don't have stroke color. when ai file printed pdf file, overlapping shapes somehow sliced or clipped , boundaries can seen.

 

example:

1) draw large, light yellow rectangle on artboard, 100% opacity

2) draw smaller black rectangle within boundaries of yellow rectangle

3) draw black circle overlaps black rectangle

4) group black rectangle , black circle together

5) assign group's transparency to, let's say, 80% opacity

5) print artboard pdf

 

after printing artboard pdf, you'll notice circle's contour can seen, although seamlessly blended black rectangle on artboard. if open pdf illustrator, you'll see there's hole on black rectangle black circle is.

 

is there way prevent happening?

what version of pdf? 1.4 or above? or did use pdfx-1a

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