Not planning on downgrading librsvg however, as I need ABIs for EFL that are in 2.51.x and above.

Updated on GitHub, but this is almost certainly a bug with librsvg rather than numix-icon-theme; the height and width attributes are optional in the SVG spec (default 100%, with us controlling size via viewbox). We can add a width attribute to any SVG file missing them in Numix to work around the problem for now, but that should only be a temporary fix until librsvg fixes whatever the problem is on their end.