No, Custom does not appear at all. I just had an email exchange with Graphisoft support where they confirmed the bug and are passing it up the chain. Here's the response I got:
Hi Rebecca,
I was able to download and dig in to your file. I'm definitely going to need to pass this up to HQ as I was unable to restore the typical functionality with various tests (share to BIMcloud and save back out, open with repair). It appears that there's something tied to 'Building Scale B' that keeps leading back to it after changes else where are made. The one helpful thing that I found is that if you go to Rename any of the other options, and complete the rename, then aside from the rename also happening you'll see a 'Custom' item does appear at the top. You can then modify all the 8 dim items to what you want and 'Save as' properly. This should represent a work around for your problem and allow you to continue using one of those newly created dim styles.
Once HQ gets back to me with more information, then I'll pass it on to you. But this will likely take them some time, and if it is a bug, then it won't get picked up until a future release. Currently, my belief is that it is file specific, so I wouldn't use this file as the source for creating others in the future.
