Ah, didn’t anticipate this.
In newest version of GP Premium, the generate_posted_on() function is used to remove the byline elements completely instead of hiding them if that option is set.
To override it further with your own function, you’ll have to add it into a custom plugin.
You can use this plugin to create a plugin: https://wordpress.org/plugins/pluginception/
Hmm, this is probably going to annoy a lot of people when they update.. Wonder if I can find a better way.