Sure it would be cool to see this in the ACF core – but it’s also nice to keep the core plugin from getting too bloated – plus it would have to be implemented in an optional/backwards compatible way – I would not want clients who have access to HEX color pickers to suddenly be able to add transparency on the next update.
Have you tried https://wordpress.org/plugins/acf-rgba-color-picker/ to fill this need?
