Good question. It definitely shouldn't do that. Itend to think that it's one of those cases where the infor from the CoDec sub-panel isn#t updated correctly. There's a couple of bugs in there. I would try to render an intermediate fiel and encode it in AME or another tool. Otherwise perhaps they may accept a plain H.264 just as well.
what Ae version? MXF OP1A was removed from Ae in 13.6 (one of the CC2015 updates - you should use ALWAYS use the most recent update of each release). I am guessing there was a reason for it, maybe it was buggy. if you are using CC2015, you should use 13.7.1 on PC and 13.7.2 on Mac.
you can find MXF OP1A in AME, try encoding through it and see if you still experience problems.
When you run into problems with the specifications from a client the best first step is usually to ask the client if they have ever had any problems receiving footage with their specifications and what they use in house. I have not tried to reproduce your problem and Mylenium is correct when he says "It definitely shouldn't do that" but I can't tell you how many times I've received production specifications from clients that have had errors. You'll never look stupid asking for clarification.
XD cam is definitely gone from a basic install of the latest versions of AE. In the old days you had to download an interpreter from Canon to use XDCamHD to both read and render to that format. It's an odd format in the way it handles audio and a bunch of other things and It's a little surprising that a client would pick it as a production standard so maybe you should be having a deeper discussion with your client.
Were you ever able to export Christian? Please let us know or if you still need help.