Not sure I'll be much help but I found myself with a similar [but not identical] question a little while back. On the WISC-V we had VCI subtest scores of 19, 19, 18 and a 12 [might have been 11?] on Comprehension. It was really odd. Our child's alphabet soup includes: E/PG, ADHD, Anxiety and Stealth Dyslexia.

I then found the document linked below which sent me down a rabbit hole I'd thought we'd ruled out - ASD+LD.

http://images.pearsonclinical.com/images/assets/WISC-V/Q-i-TR11_WISC-V_ADHDAUTL_FNL.pdf

But we had previously ruled out ASD so this really threw me for a loop but our clinical psychologist firmly insists that we are not dealing with ASD at any level on the spectrum [rationale: TOM and perspective taking are in-line with cognitive age which means it is years ahead of actual/chronological age].

Ultimately, the psychologist feels that the low comprehension score is symptomatic of the ADHD and Dyslexia ... the neurobiology of those differences combined with age [6] and extreme asynchrony make for delayed development of this skill. The psych believes that once the dyslexia is treated the comprehension will follow [yes, even though the comprehension subtest is presented orally, the psych believes it is all tied together somehow].

I'm not sure that will help but thought I'd share in case you notice any similarities. Have you read much on Stealth Dyslexia? If not, this will get you started ... I know you said your child is a voracious reader but that does not rule out Stealth Dyslexia so definitely check it out! Just in case smile

http://www.davidsongifted.org/Search-Database/entry/A10435