Unfortunately, IB is correct in this case (and this is a standard rule for most airlines):
[KVS Availability Tool 5.1.1/Diamond - DotRes: BLQ-BCN/IB/P14NEOI4/66 USD]
Just out of interest, what part of the text that you have copied says this? I have gone through it, but could not find it. I guess it’s somewhere in “COMBINATIONS”...
I would like to know this because then I can look for it when booking future travel. I also sometimes book different fare classes on the same ticket. (I know that for instance Brussels Airlines applies the correct rules for flexibility for each leg. I have used this in the past without any problem.)