In my actual "problem case" I was using only weightmaps, not zones.

Don't the connected actors have to rotate around the same reference point? Otherwise the bend transformation cannot be steady?

If the "center" in the twistX is not referring to the joint center - how can weightmapped actors rotate around different centers, how can that be possible :(

I had the connected actors entirely weightmapped to 1.0, and the bend ripped them apart. I observed one was rotating around the morphed joint center, the other was rotating around the unmorphed joint center.