Hi!
Thank you very much, I was able to fix the issue.
There were two problems:
1. The code was incorrect, I made a mistake
2. The code was not active, since the child theme is not active
I corrected the code, and put it into the active theme directory functions file instead of the child theme file.
For future reference, the corrected version is:
add_filter('asp_query_args', 'asp_query_args_meta_changes', 10, 1);
function asp_query_args_meta_changes($args) {
$args['user_search_meta_fields_separate_subquery'] = 1;
return $args;
}