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Depending on how one interprets "using the same syntax of the TinyMCE valid_elements option", this may actually be a feature request.
The TinyMCE valid_elements documentation states for the "/" character: "Separates element synonymous. The first element is the one that will be output."
I take this to mean if you have a definition like "abbr/acronym", both elements are allowed, but when the page is actually rendered, both elements render using the first element. What is happening instead is that both elements are allowed, and are rendering as themselves.