I'm surprised their beta testers did not run into this in the wild. That tells me it was probably a last minute change that somehow get added that didn't quite get cleaned up in the Gold Master. . . Probably a commented command that did not get deleted and re-instated on final after testing in a laboratory setting where there was no handoff taking place.
I expect Apple will send out a fix next week if not sooner. This is the kind of handoff that is handled routinely with the iPhone and LTE iPads multiple times a day, so the code is boilerplate. It just needs to be bolted in place in the watchOS.
I”m not so convinced that it is so “routinely handled” by iPhones... My last three (current 7+, previous 6+ and 6) all fumbled pretty badly when handling off from wifi to LTE. And recent iOS versions have been somewhat frustrating - about 25% of the time I have to turn off wifi to force it to make the transition to LTE. Rather frustrating.