Sorry to hear this is also a problem with no multi-item defaults. I have submitted to FB15267454.

Your suggestion to look at the raw .plist XML was interesting. I also tried this and manually keyed in a Multi-Value option and it appears to work now? I slowly copy-pasted from an older plist to the new one and was able to get them all working. Unclear if it is a legacy project issue in Xcode or something else, but I can confirm that I was able to get multi-values working by editing the raw file.