The solution was: one has to use the filter string from the $t_filter
array.
# get filter string
$t_filter_string = explode('#', $t_filter['filter_string'], 2);
# get bug rows with unserialized filter string
$t_rows = filter_get_bug_rows($f_page_number, $t_per_page, $t_page_count,
$t_bug_count, unserialize($t_filter_string[1]),
helper_get_current_project());
So the wanted rows are returned and additionally the current project is used to filter.