Yup, for sure, intermixing polymer sections can cause a cut thru as described. I first ran into this in the 70's when Nylorfi came into the market. On its own, it was way stronger (except for wind knots) than the nylons from Cortland, Kroic, Orvis, etc.
I don't know if the marketers were evil enough to predict that having the thinner, presumably weaker section not break before the fatter, softer Brand X would only confirm in your mind how weak the old fashioned nylon was.
The cut thrus often didn't occur until you had been ffishing for a few hours, and then just touching a blade of grass on a backcast would part the leader right above the blood knot. A long fight or 2 on big trouts or too many yanks out of a tree would have the same effect.
The other subtlety is the knotted leader might be just fine until it has gone through a few uses and rests. Nylon, and copolymers that incorporate it, dimensionally change a few % as they soak up water (and release it when the leader is stored in a dry place), and this cycle gradually weakens the knot.
The stiffer monos do tend to cut into the limper ones. Frog Hair isn't the only culprit; Maxima in any color can behave similarly. I do mix and match a lot, especially since I repurpose a lot of abandoned leader material and spinning line. I protect myself by keeping the 8X-5X parts of the leader the same brand/model.
The thicker the tippet diameter, the less I notice the cut thru problem.
tl
les
