Hi Vincent,
Are you aware of the setting ‘Show all results when sorting. Default is off.’ on our settings page?
For boolean field and more specifically ACF boolean fields you have the following options
empty = True/False (determined by user)
0 = False
1 = True
So if you sort on a boolean field, it tries to sort on 0 and then 1 but it with the setting above off, it ignores all records without any value. This is intended because the Custom Field column is made for all custom fields (not just ACF) and we simply don’t know how an empty value should behave (true or false).
Since you’re using ACF you could try our ACF addon for Admin Columns Pro (business license and up) because for ACF users we already know more about the context of your fields and we know how empty values should be handled. So the we believe both filtering and sorting does work as expected for the Custom Field column as this is a solution for all custom fields implementations.