We experienced the same issue on several of our projects, and tracked down a recent change in an internal dependency as the source of the issue.
The grunt-contrib-cssmin module has an internal dependency on the clean-css module. Starting with version 3.4.1, the clean-css module turns on unit conversions by default, which then bubbles up into the behavior of grunt-contrib-cssmin. The clean-css module released version 3.4.2 which changes the default back to turn off unit conversions, based on community feedback. Version 0.14.0 of grunt-contrib-cssmin has been updated to use version 3.4.2 of clean-css.
We were able to resolve this issue by updating to use version 0.14.0 of grunt-contrib-cssmin in our project's package.json file. Make sure to clear your node-modules folder or run 'npm update' to ensure it's actually getting the latest versions.