Hello @JPhil - The PolyCurve.GroupByCurves node will return the exact same results as Polycurve.ByJoinedCurves - it’s just smarter about how to generate those 
So that will not be the problem.
I suspect either that particular set of input curves have something at fault, or something has changed in the underlying API being called.
Can you try with a simple use case? i.e. a line? Do you get the same error?