Knot doesn’t delete zonefile after de-catalog member zone

After de-catalog member zone there is a message „zone purged“ in logfile, but even if zonefile-sync is default or explicitly set to 0, zonefile is not deleted.

After readd member to catalog, the old zonefile is loaded with old serial. We've noticed it in Knot 3.1.8, but probbably older version are affected too.