I believe we may have found an issue with the way @embed is replaced in less files. According to the comments and code in CSSMin.php (around #313) @embed should not have the fallback for old IE versions added when the file is an SVG. However, we've found that this is exactly what happens - for example, see Vector's generated css.
This is the case in 1.25 and core.
Thanks to @connorshea for noticing something weird in the first place.