Hi Tim,
I am assuming you are using the {get_values} syntax within the filter to automatically get the checkbox values.
The problem here is, that there are no pre-defined values within the ACF field group, you simply enter the values as-is into a text field. If you were to use an ACF checkbox, then the plugin would be able to parse the individual values only, as those values are stored before using them.
I think there are two possible ways to resolve this:
1. To use a static checkbox ACF field instead of the text field
2. ..or simply enter the author names manually to the filter, instead of using the {get_field} syntax