IXFR and lost (?) updates of a TTL

[Stephane Bortzmeyer]

We experienced a curious BIND bug on slave machines which use IXFR to
transfer from a BIND master. The TTL on the NS records were
changed. While the master had:

$TTL 172800     
wf                      IN SOA  nsmaster.nic.fr. hostmaster.nic.fr. (
                                2006022800 
                                3600       
                                1800       
                                3600000    
                                5400       
                                )
                        NS      a.ext.nic.fr.
                        NS      a.nic.fr.
                        NS      b.ext.nic.fr.
                        NS      b.nic.fr.
                        NS      c.nic.fr.
                        NS      d.ext.nic.fr.
                        NS      e.ext.nic.fr.
                        NS      e.nic.fr.

The IXFR slaves had:

$ORIGIN .
$TTL 172800     ; 2 days
wf                      IN SOA  nsmaster.nic.fr. hostmaster.nic.fr. (
                                2006022800 ; serial
                                3600       ; refresh (1 hour)
                                1800       ; retry (30 minutes)
                                3600000    ; expire (5 weeks 6 days 16 hours)
                                5400       ; minimum (1 hour 30 minutes)
                                )
$TTL 345600     ; 4 days
                        NS      a.ext.nic.fr.
                        NS      a.nic.fr.
                        NS      b.ext.nic.fr.
                        NS      b.nic.fr.
                        NS      c.nic.fr.
                        NS      d.ext.nic.fr.
                        NS      e.ext.nic.fr.
                        NS      e.nic.fr.
[Note the spurious $TTL.]

'rndc retransfer $ZONE' fixed the problem but I wonder if it may comes
back. Can ".jnl" files get corrupted? Any tool to scrutinize them?

The slaves which used AXFR (like a nsd box) were not affected.