I am facing this issue in PI 7.1 and I beleive i did similar one in PI 7.0 successfully.
I have an operation mapping with two message mappings.
The occurence of source message of operation mapping is 1
The occurence of target message of operation mapping is 0..unbounded
Corresponding i configured two message mappings
For message mapping-1
Source message occurence = 1 and Target message occurence = 1
For message mapping-2
Source message occurence = 1 and Target message occurence = 0...unbounded
When i test message mappings individually they are working good. But when i test it in operation mapping, the operation mapping is not working..as it is not able to convert single occurence output of message mapping -1 to the second message mapping-2.