librsvg has broken attribute handling for masks
If an attribute is applied to an element that uses a mask, and the mask itself contains elements for which that attribute is unset, that attribute will be unexpectedly appear to be applied to the elements inside the mask. This behaviour is inconsistent with the behaviour of other SVG renderers, including those of Firefox and Safari.
One example is File:BSicon tvÜWBrq.svg at Wikimedia Commons. The first version and the current version of the file render identically in Firefox and Safari, but the librsvg thumbnails (1, 2) are different.
I first reported this as task T229666 at phabricator.wikimedia.org. There is some further detail in the task description and comments.