Prevent site history report from globally altering custom_field_preprocess on ReportView

wagtail/admin/views/reports/audit_logging.py

@@ -95,6 +95,7 @@ class LogEntriesView(ReportView):
     def __init__(self, **kwargs):
         super().__init__(**kwargs)
 
+        self.custom_field_preprocess = self.custom_field_preprocess.copy()
         self.custom_field_preprocess['action'] = {
             self.FORMAT_CSV: self.get_action_label,
             self.FORMAT_XLSX: self.get_action_label