TYPO3 Core

btw: this also effects the fix of relative url paths:

TYPO3\CMS\Core\Resource\ResourceCompressor Line 356

        if (!file_exists(Environment::getPublicPath() . '/' . $targetFile) || $this->createGzipped && !file_exists(Environment::getPublicPath() . '/' . $targetFile . '.gzip')) {
            $contents = $this->compressCssString(file_get_contents($filenameAbsolute));
            if (strpos($filename, $this->targetDirectory) === false) {
                $contents = $this->cssFixRelativeUrlPaths($contents, PathUtility::dirname($filename) . '/');
            }
            $this->writeFileAndCompressed($targetFile, $contents);
        }
        return $this->returnFileReference($targetFile);

this was an side effect of compression but comes in really handy for inline css.

To fix this issue some more refactoring is necessary because TYPO3 dose not difference between gzip and whitespace compression this is done one process flow.

As well there should be an option to run cssFixRelativeUrlPaths without compression.