SVG and PNG math renders in the MathML render mode are pulled in as background images, so that browsers with MathML support do not load SVG fall-back images. However, browsers default to printing without background images, which means that formulas go missing when printing web pages using default settings, which default to omitting backgrounds. Users can work around this by manually checking "print background images" in the print settings, but it is likely that they won't notice the missing formulas before holding a print-out in hand.
There is a CSS work-around for Webkit, which can be targeted at math elements: `-webkit-print-color-adjust: exact;`. This makes Webkit print background images in the element this style is applied to, regardless of print settings.
However, it is not clear if there is a solution that will work reliably for other browsers. http://maxlapides.com/forcing-browsers-print-backgrounds/ discusses several options, but we'll have to test whether any of them can be made to work reliably for math. Formulas seem to be missing in print-outs even when using Firefox with the MathML plug-in.
If no work-around can be found, then this brings us back to the perennial question of whether MathML is really worth the hassle. If we dropped MathML in favor of regular SVG images, then this problem would go away.