One of the possible solution for this
Go this file:
['a', 'href target rel media hreflang type charset name rev shape coords download', phrasingContent],
['a', 'href target rel media hreflang type charset name rev shape coords download', flowContent],
Good approach to override this file in your custom theme
The reason this fixes the issue is that the
flowContent array includes block level tags and inline tags unlike the
phrasingContent array which includes only inline elements.
The first element of this array is the a the tag we want to fix, the second array entry appears to be attributes allowed on a tags, and the third array entry (the one we care about) appears to be allowed children.
If you check out to the TinyMCE 3 configuration archive above, there are some settings which would allow you to disable it, for example this one:
I hope this will help