Regression when SVG has only one of width/height and a viewBox

After the weekly update of Solus Linux a few days ago, the User Indicator icon (Numix/scalable/actions/system-shutdown-symbolic.svg) had disappeared both from the Budgie Panel, as well as the LightDM login screen, yet not from the file system. It has since been determined that a downgrade from librsvg-2.52.0 to librsvg-2.50.7 fixes the problem. Another fix was to insert width attribute in the SVG file which, according to the SVG standard, is entirely optional. This might hint at a bug in librsvg-2.52.0.

The wrong behavior occurs when a viewBox attribute is present and additionally either one of width and height is present, but not if both are present or if both are missing. See Joshua Fogg's chart below.

The SVG version 1.1 standard specifies for both width and height that if either is missing, it is treated as having a value of 100%. Version 2 instead specifies that "a computed style of auto is treated equivalent to 100%", whatever that means. I wasn't able to figure out the precise meaning of "computed style", since that phrase seems to be used only once in the whole standard document.