Hi,
Thank you very much for all the details, I have found the problem. The issue was, that the custom field is not ‘_nickname’ but simply ‘nickname’: https://i.imgur.com/l4PAq8Z.png
ACF usually creates two fields, one prefixed with ‘_’ for reference only.
I have selected that field as well, and now if I try to search for a nickname, it seems to be returning the expected results now.
