I may have figured it out, the Latex formula requires the \begin ... \end, which were missing above, when added the begin-end, the formula renders correctly!
One more thing - this problem appears only when selecting the "Latex" equation type, but not in "Display" or "Inline"
\frac not rendered correctly
As you say "there are many things to do" to install KHC on Linux. I wonder, is there...