SVG generated in mathml mode comes with a style attribute of the following form:
style="vertical-align:-.505ex" (there is no closing semicolon).
When this attribute is processed in function correctSvgStyle it sticks to the next attribute (height).
That leads to the following HTML code:
<meta class="mwe-math-fallback-image-inline" aria-hidden="true" style="background-image: url('URL;mode=mathml'); background-repeat: no-repeat; background-size: 100% 100%; vertical-align:-.505exheight: 2.843ex; width: 28.527ex;">
Such a picture has zero height and thus is invisible (in chrome).
I fixed this issue with a following code in MathML.php:
$style .= ' ' . $styles[1] . '; ';
instead of
$style .= ' ' . $styles[1];