This sort of thing also occurs with the round reducer with a branch (forgot the CID), the secondary diameter could not be equal to or larger than the primary which didn't make any sense. I beleive this is just a limitation with the ITM file itself, or made that way. So I'm afraid I can't offer any assistance.

There are issues with a lot of ITM's and very little consistency with the way they were first made (I don't believe Autodesk has made more than a few). This won't change until Autodesk either fixes the ITM's, or they give us the software to make our own. I'd much prefer the latter option but I'll take anything at this point.