Thanks Greg. 

Much appreciate you takling the time to help out.

You are right, with waveguides this works fine and. interestingly. if you put another MZI down stream of this that also works fine? So it does seem to be related to the built in fibre optics elements in Interconnect and like you say "the lack of change is FSR would be consistent with a missing group index in fibres"? 

I see no way to set the Group indexattribute on the built in fibres. Note I get the same result on all three built in fibre types.

So maybe this is just something that Lumnerical needs to update?