Thanks for your response Mylenium. I'm not knowingly using any blending effects. My raster effect settings are:
Attached are two screenshots to illustrate my problem. The first shows the AI file open in Illustrator with the correct order, the second shows the result of exporting to PNG. Notice that the error bars are brought forward in order during export.
Figure 1 (above): the AI file open in Illustrator with the correct order.
Figure 2 (above): the PNG file after exporting. The export settings are as follows:
Use Artboards (all)
Resolution: High (300 dpi)
Anti-aliasing: Type optimized (hinted)
Background color: White
I tried setting the raster effect setting resolution to be the same as the desired export resolution, but that did not solve the problem.
Thanks again for your help!