lists.isc.org rDNS failed, DNSSEC?

[/dev/rob0]

Yesterday I looked in mail logs for something else and stumbled upon 
this (times are UTC):

rob0 at harrier:~$ grep 'unknown\[149\.20\.64\.75\]' /var/log/maillog | wc
    271    3607   44087                                                                
rob0 at harrier:~$ grep 'unknown\[149\.20\.64\.75\]' /var/log/maillog | head -1
Feb 21 05:28:25 harrier postfix/smtpd[4653]: connect from unknown[149.20.64.75]
rob0 at harrier:~$ grep 'unknown\[149\.20\.64\.75\]' /var/log/maillog | tail -1
Feb 21 21:32:06 harrier postfix/smtpd[3575]: disconnect from unknown[149.20.64.75]

During that time I tried a "dig 75.64.20.149.in-addr.arpa. any" and 
got SERVFAIL. I checked 64.20.149.in-addr.arpa at Sandia's DNSViz, 
and it was fine. I was in a hurry so I didn't think to check 
75.64.20.149.in-addr.arpa. I whitelisted 149.20.64.75 so this list's
mail would come through; went on with other things.

I was thinking that the problem might have been on my end, but I 
changed nothing before nor since; 75.64.20.149.in-addr.arpa/IN/PTR 
since 21:32 UTC yesterday has been returning "lists.isc.org."

Any idea (especially from ISC folks) what might have caused this?

This is the scary thing about DNSSEC: a lot of valid zones might 
suffer from temporary glitches wherein signatures fail. I know one of 
my own zones had expired signatures awhile, and I have seen it with 
subscribers on my own Mailman lists.
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