It's very appealing to smugglers because its ultra-high potency means that a massive amount of doses is very, very tiny.
Thus, no surprise that it shows up elsewhere.
Thus, no surprise that it shows up elsewhere.
The black market is very inconsistent with what it provides, (because it is unregulated,) thus "heroin" may have fentanyl in it; or may not even be available. (Because it's easier to smuggle fentanyl.)