Yes, inadequately-managed ADHD can lower cognitive assessment scores. Substantially, for some persons. I can't comment on whether the current scores are valid or representative of your DC's underlying cognition, but it is certainly worth investigating. If you do choose to treat in some way (pharmacological or otherwise), and to retest, be aware that the recommended retest wait period for the WISC-V is 24 months, so a follow-up cognitive assessment during that window would have to be with a different instrument (e.g., KABC-II NU, WJ-IV, DAS-II, SB5, in order of currency of norms).
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...pronounced like the long vowel and first letter of the alphabet...