Observed stats underflow in multiple stats

While looking at a customer-provided named.stats file it was observed that several counters had underflowed in BIND version 9.11.16-S1.

A selection of the underflowing stats:

+++ Statistics Dump +++ (1584405000)
++ Cache DB RRsets ++
[View: default]
18446744073709551614 ~A
18446744073709551615 ~NS
++ Socket I/O Statistics ++
18446744073709551556 UDP/IPv4 sockets active
18446744073709551584 UDP/IPv6 sockets active
18446744073709551600 TCP/IPv4 sockets active
+++ Statistics Dump +++ (1584405300)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~CNAME
18446744073709551614 ~!AAAA
+++ Statistics Dump +++ (1584405900)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~AAAA
+++ Statistics Dump +++ (1584408000)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~RRSIG
+++ Statistics Dump +++ (1584418500)
++ Resolver Statistics ++
[View: default]
18446744073709551615 active fetches
+++ Statistics Dump +++ (1584447000)
++ Cache DB RRsets ++
[View: default]
18446744073709551608 ~NXDOMAIN
+++ Statistics Dump +++ (1584485100)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~NULL
+++ Statistics Dump +++ (1584504300)
++ Cache DB RRsets ++
[View: default]
18446744073709551521 NULL
+++ Statistics Dump +++ (1584515400)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~TXT
+++ Statistics Dump +++ (1584518400)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~NSEC
+++ Statistics Dump +++ (1584676500)
++ Cache DB RRsets ++
[View: default]
18446744073709551557 !AAAA
+++ Statistics Dump +++ (1584737700)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 !CNAME
+++ Statistics Dump +++ (1584886800)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~MX
+++ Statistics Dump +++ (1585119900)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 !MX
+++ Statistics Dump +++ (1585137000)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 ~DS
+++ Statistics Dump +++ (1585282500)
++ Cache DB RRsets ++
[View: default]
18446744073709551610 !A
+++ Statistics Dump +++ (1585457400)
++ Cache DB RRsets ++
[View: default]
18446744073709551506 CNAME

I'd like to specifically call out active fetches which might get lost in the noise near the middle of those lines.

The same stats file has history going back to unknown prior versions (August 2019) also has underflows on:

+++ Statistics Dump +++ (1566255596)
++ Cache DB RRsets ++
[View: default]
18446744073709547273 #A
18446744073709551608 #TXT
18446744073709551247 #AAAA
18446744073709551615 #SRV
18446744073709548950 #!AAAA
+++ Statistics Dump +++ (1566260101)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #NS
+++ Statistics Dump +++ (1566307500)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #RRSIG
+++ Statistics Dump +++ (1566355500)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #!A
+++ Statistics Dump +++ (1567977300)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 !A6
+++ Statistics Dump +++ (1568136600)
++ Name Server Statistics ++
18446744073709551466 recursing clients
+++ Statistics Dump +++ (1570120200)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #!SRV
+++ Statistics Dump +++ (1571707501)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #CNAME
+++ Statistics Dump +++ (1571768400)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #!TXT
+++ Statistics Dump +++ (1574442900)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #DS
+++ Statistics Dump +++ (1574886601)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #NXDOMAIN
+++ Statistics Dump +++ (1575558600)
++ Cache DB RRsets ++
[View: default]
18446744073709551615 #PTR

The most significant of the last set is probably recursing clients.