BIND server; dig vs dig +trace on failing lookup.

[Gregory Sloop]

I've got a case, (and I see several other similar reports) where BIND is failing to find an A record for a domain.
Yet a dig +trace does.

(I'm doing the dig on the BIND server. It's set to be a root resolving server, not a forwarder.)

As I understand this, +trace will also involve resolve.conf options. And in this case, I've got Google DNS as one of the resolve.conf entries.
So, I can see how +trace would deliver different results than simply dig-ing - provided that +trace does involve resolve.conf.

Here's a plain dig, using the BIND server itself - from the console.
---
dig cistus.com @10.8.20.5

; <<>> DiG 9.11.3-1ubuntu1.14-Ubuntu <<>> cistus.com @10.8.20.5
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 61786
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 13ec0c9b10770ea12426539e603957900a997f7258962cce (good)
;; QUESTION SECTION:
;cistus.com.                    IN      A

;; Query time: 0 msec
;; SERVER: 10.8.20.5#53(10.8.20.5)
;; WHEN: Fri Feb 26 12:18:24 PST 2021
;; MSG SIZE  rcvd: 67

---

I could post the dig +trace, if it adds any information, but I suspect it doesn't.

So, what methods or steps might I take to figure out why the above lookup/dig fails? 
[I intended +trace to do that, but since it's not doing the same thing a plain dig does, it's not very useful as a diagnostic tool.]

I've done some searching to see how to accomplish this, but it's a difficult question to frame without a ton of worthless hits.
So, can someone point me at a good source for a how-to/walk-through? A previous list posting?

Again, the question is; what methods or steps (best practices) might I take to figure out why the above lookup/dig fails?

TIA
-Greg
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