SelectFilter->getOptionLabelFromRecordUsing
The label isn't applied either
I even tried to dd() in
getOptionLabelFromRecordUsing() but it doesn't get called at all

Solution
Likely because your code is wrong. $actor won't exist...
Why not do:
Providing the model of the resource has the user relationship?
Why not do:
Providing the model of the resource has the user relationship?