cell formulas aren't calculated in the the html output (only the formula definition is displayed).
It is supposed to display the result of the calculation, not the formula, with some exceptions (imposed by the POI project...) : It can not
process custom/user formulas, as it doesn't know how to handle the VBA involved. And it won't until someone implements a VBA interpreter in Java
There are more restrictions on formula's as described in this Apache POI page
I tested with quite simple '=J2+J3' etc. formula's (that didn't get into the demo data, but will be in the next update) and that works just fine.
Can you give an example of a formula (and an expected result, if possible) that doesn't compute?
I did some investigation, and it seems I need to re-evaluate all formulas when reading them. That requires another library jar (ooxml-schemas instead of the much smaller poi-ooxml-schemas) so the download will increase quite a bit. I'll release v126.96.36.199 shortly after this message (v188.8.131.52 is already available with just minor changes, but I didn't have time to write the announcement).