So, yeah, insulin requirement, on the one hand, is dependent on Total Available Glucose from carbs and gluconeogenesis from protein. When you drop carbs to Bernstein levels, that's a significant decrease in the requirements for insulin to cover your TAG. There's also the law of small numbers that says that smaller insulin boluses from less TAG act much more predictably than larger doses from more TAG.
Hopefully you've had these positive outcomes.
On the other hand, IC ratio is a separate consideration. Lot's of things also affect IC ratio, such as lean body mass, and activity level, and insulin resistance.
Have you changed anything else besides your TAG and fat intake?
Fat does have a pretty well known habit of slowing glucose absorbtion so that correctly calculating your TAG still results in a big spike, except that the spike occurs much later than usual (the "Pizza Effect").
Is your increased IC ratio based on high BGs occurring later than usual?
There is also this discussion in the general forum:
https://forum.tudiabetes.org/topics/dietary-fat-can-affect-glucose-and-insulin-needs-of-type-1s
Not to suggest that fat is bad for you, but I know that no matter how accurately I calculate my TAG based on my own IC ratio, and no matter how many extended boluses I program into my pump I always have to give myself more insulin to cover a high fat meal than a lower fat meal, all else being equal.
If fat is responsible for increasing my IC ratio, independent of TAG, so be it. Doesn't have to change my diet in anyway, just the way I calculate my insulin dose. As long as my carb and fat content are to my satisfaction, and my insulin dosing, overall, is satisfactory. Ultimately, it's all about BG control.