I think I figured this out.
There is a default internal dns-proxy object called mgmt-obj that works to resolve hostnames when you check the "resolve hostname" checkbox in the various monitor logs. To do this it does a reverse dns lookup using the arpa database - and sends a dns query for a pointer record of the domain name.
ie - if one of your devices/apps was reaching out to a.b.c.d and your "monitor traffic" gui pane had the resolve hostname checkbox enabled - to facilitate this, the PA device would send out a dns query for a pointer record of the domain name d.c.b.a.in-addr.arpa to your specified dns servers. If this resolve fails ... it logs it in the system log as type: dnsproxy, severity: informational, event: resolve-fail, object: mgmt-obj and provides a description of the failed reverse lookup attempt
I actually created a support ticket for this issue. 8.1 was when I first saw the issue as well. I have been told that this is a know bug with 8.1 and it will be addressed with 8.1.1. 8.1.1 should be available around May 3rd. Here is the bug id for this issue:
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