Hi,
First of all, I would like to say that WordPress does not have official way to make css asynchronously, there are only a few workarounds, for example https://gist.github.com/schilke/02357d9263ed28fc1769
According to this, we did not add such unofficial solutions, as there are no guarantees that this will work as it should, without any issues on the customer sites.
I think, in the future versions of the plugin, we will investigate this problem more, but at the moment, I recommend using special plugins (which is very much) that are sharpened precisely for these purposes, for instance:
https://www.ads-software.com/plugins/wp-fastest-cache/
https://www.ads-software.com/plugins/autoptimize/
https://www.ads-software.com/plugins/async-js-and-css/
https://www.ads-software.com/plugins/async-javascript/
https://www.ads-software.com/plugins/speed-up-optimize-css-delivery/
Thanks!