AI cities that become new capital, after they lose their original capital, cannot be razed. This is clearly a bug, as the civilopedia says under the entry <Current Capitals vs. Original Capitals> "If your original capital has been captured, another of your cities will automatically be assigned as a replacement capital. Your Headquarters will be transferred automatically and this city functions in all ways like the original, except that it can be destroyed". Also you could destroy new capital cities in original BE.
This is not a one time thing, this has been happening with me 100% of the time since the launch of Rising Tide.
This is not a one time thing, this has been happening with me 100% of the time since the launch of Rising Tide.