A formula's html is the markup comark-mathml typesets from TeX, so the renderers print it as it is. A :formula written by hand with its own html is ignored, and it shows its tex, if it has one. Write the TeX between $ signs instead.
Incorrect
The pond holds ducks.
The pond holds :formula{html="<math><mi>n</mi></math>"} ducks.Correct
The pond holds ducks.
The pond holds $n$ ducks.