Compare the two versions of File:BSicon tvÜWBrq.svg. In Firefox and Safari they are identical, but librsvg does not render the older version correctly (PNG thumbnail of the older version; thumbnail of the current version).

As far as I can tell, this is because librsvg incorrectly applies styling from parent elements onto the elements inside a mask (due to the mask being used somewhere inside the parent element), even though the expected behaviour is that only the elements that are actually inside the parent element should be styled. I believe this may be related to a number of other SVG mask-related issues, including T55899 and T46016, and it has caused issues with several other BSicons (I have had to fix at least a dozen of these over the last few years, but I don't think I filed a bug report).

What appears to be happening in the older revision of the file is that the attributes stroke="#BE2D2C" stroke-width="100" fill="none" are being incorrectly applied to the constituent elements of mask 2. As a result, a faint red ring can be seen in the middle of the older thumbnail (the red parts are where the ring overlaps with the first <path/> element). In the current version, the latter two attributes are still being (incorrectly) applied, but because the stroke attribute is undefined the ring is not rendered.

In terms of the actual SVG code, the issue is caused by the nesting of <g mask="url(#2)">...</g> inside <g mask="url(#1)" stroke="#BE2D2C" stroke-width="100" fill="none">...</g>. The attributes inside the parent <g/> should only be applied to <path d="m 0,0 m 500,0 M 125,-25 V 0 c 0,250 250,250 250,500" stroke-dasharray="50,46.291"/>, and not to the children of <mask id="2">...</mask>. Copying stroke="#BE2D2C" to both <path/> elements and removing it from the parent <g/> element is enough to resolve the issue, but this is reliant on the stroke attribute not being set for the second circle in mask 2.