I am having the same problem. Currently on hold with support and will post back what I find.
Before I got on with support, I found William Lam's blog post resolving the problem:
New method of enabling Multiwriter VMDK flag in vSphere 6.0 Update 1 (UI + API) | virtuallyGhetto
Quick sum up: They changed how things are done in 6.0. Use the web gui, or set the new disk backing prompts with @lamw's powercli.