Hey Eric,
Thank you so much for exploring this and for sharing your findings. That DB_CHARSET definition is an interesting one and if that resolves things I would definitely recommend proceeding with that. I do not know of any ill side-effects from this, and in fact the WordPress change to utf8mb4 is a fairy recent one, so there shouldn’t be any fallout.
Thank you for your patience and persistence here. Let me know if there is anything else I can try to help with!
George