The High C4 read in Manuka honey is a known issue intrinsic to the honey, per research published in New Zealand.
The Unique Manuka Effect: Why New Zealand Manuka Honey Fails
the AOAC 998.12 C‑4 Sugar Method ,   Karyne M. Rogers

Open access:  https://www.researchgate.net/profile/Karyne_Rogers/publication/259845309_The_unique_manuka_effect_Why_New_Zealand_manuka_honey_fails_the_AOAC_99812_C-4_sugar_method_-_Part_2/links/59e42fb50f7e9b97fbeb0f6b/The-unique-manuka-effect-Why-New-Zealand-manuka-honey-fails-the-AOAC-99812-C-4-sugar-method-Part-2.pdf

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