All I can do is guess the usual suspects, cause it's not something we can test for. But my guesses are one or both of glucose toxicity and/or IAA. His body is not so much responding to insulin increases, but to lack of time spend in normal blood sugar range. His body is used to higher numbers and wants to hang out there.
Well, a sliver of a trend down but not much. The question becomes: What can one do about glucose toxicity if that's the case? Like I read this "If hyperglycemia (216 mg/dL; >12 mmol/L) persists for longer than 4–6 hours, low-dose insulin therapy (e.g., 0.5–1 U/cat or 0.2 U/kg q 12 hours) should be given. Exogenous insulin lowers blood glucose and helps overcome deleterious effects of hyperglycemia on β-cell function." Jun 19, 2018
Essentially what needs to be done is to increase until kitty sees enough normal BG numbers that they get used to them.
I was just doing a search for Glucose toxicity cats and kept getting that kind of information.I'm not sure what document you are referring to, the dosing maybe refers to starting insulin? See this post: Feline Diabetes and Glucose Toxicity links. Unfortunately there are some broken links on that page - another on the to-do list to fix.