Well, I think you'll find that what is meant by "backwards compatibility" doesn't necessarily mean editing R5 design elements using the ND6 client and expecting everything to be OK (I could be wrong, but I personally wouldn't do this).
Whilst the issue you are experiencing certainly sounds like a minor bug that can be addressed, what you have to bear in mind is that "backwards compatibility" equates to Notes 4.x and Notes 5.x code working, pretty much as is, in future releases. Sure, future releases add / fix features, so you should always test, but the upshot is that each major version doesn't mean a "rip and replace" strategy with your apps.
Now, tweaking design elements with a later version of the client is something I'd be inclined to steer clear of if you still have the earlier version in place within your organisation. It is also possible to run the R5 and ND6 clients on the same machine -- search the forum for more on this.
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