It's more of a limitation than a bug. Full unicode fonts are very large and probably require a lot of memory. This is not a strength of eink readers. I suspect Kindle, Nook and other eink readers may have similar limitations. You can't please everyone. Their system fonts are targeted at 'the average user' not every specialist under the sun.
However, if you buy a retail maths epub I would have to hope that the publishers have embedded a suitable font (or possibly a subset of a font) which allows your book to be read properly on all epub reading devices.
If you're creating your own maths epub, just try Googling for a free maths font and experiment with it once it's embedded (or even just sideloaded to the Kobo fonts directory).
Here's one you can try. It may not contain 100% of all possible maths glyphs but it should have better coverage than EBGaramond.