Posted 30' too early, just accidentally came on a reproduceable scenario !!!
1) In a transformer, create an Iterator over any field (e.g. PID.3) and add 2 steps within it (e.g. Message builders on PID.3.1 and PID.3.2).
2) Create a new message builder step on any other field (e.g. PID.4.1) and remove it from iterator.
3) Go back on first pid.3 substep, and reduce PID.3 view (plus/minus button) of the iterator without setting the focus on the iterator itself
4) go to the pid.4 transformation ; it should show the PID.3.1 transformation.
EDIT : something very similar
3a) While focus on PID.4.1 transformation, reduce PID.3 iterator.
4a) Now expand it and go to first PID.3.1 transformation, it should show the content of the PID.4.1 step