It's got to be the IMO from tapioca starch.
"Isomaltooligosaccharide (IMO) is a mixture of short-chain carbohydrates which has a digestion-resistant property. IMO is found naturally in some foods, as well as being manufactured commercially. The raw material used for manufacturing IMO is starch, which is enzymatically converted into a mixture of isomaltooligosaccharides."
On the Funday site in the FAQ they talk about unavailable carbohydrates.
"Some types of carbohydrates make it all the way to our large intestine which are either fermented by the bacteria that live there (which technically can provide your body with a small energy source) or passed through to the toilet. These types of carbohydrates are called unavailable carbohydrates."
I found this info on IMO's
"IMOs are 60 to 70% as sweet as sucrose (sugar), less viscous than maltose, and are partially digestible, containing about 2.0-2.4 calories per gram. [1][3][4] This article will discuss the benefits, downsides, food sources, and supplement sources of isomalto-oligosaccharides."
"
Isomalto-oligosaccharides are touted as a low-glycemic sweetener safe for diabetics but some studies found less than favorable results for its impact on blood sugar. When six fasted human subjects consumed 25 grams of IMO syrup under resting conditions their average blood glucose levels increased from 109mg/L to 136mg/dL and insulin levels increased from 4.8 ?U/mL to 32 ?U/mL 30 minutes post-ingestion. [11]
A 24.8% increase in blood glucose and 566.7% increase in blood insulin levels suggest that IMOs are far from low-glycemic. This was a very small study and we should not be surprised to find that nearly one ounce of a sweet carbohydrate on an empty stomach impacts blood sugar and insulin levels. In contrast a study on young healthy adults found that daily a IMO consumption of 15 grams did not negative impact lipid concentration and glucose absorption. [4]"
https://www.tigerfitness.com/blogs/supplements/isomalto-oligosaccharides-guide