For the moment replace “number=no” with “incrementnumber=no”.
This bug appears to still be with us. Has a solution since been proffered? If not, is a resolution in sight?There is a bug but it takes time to find it, unless it is found and fixed use incrementnumber.
BTW: A *minimal* example your problem is **this**:
\setupformulas[way=bysection,prefixsegments=chapter] %\setuphead[subsection][number=no] \setuphead[subsection][incrementnumber=no] \starttext \chapter{Chapter 1} \chapter{Chapter 2} \section{Section} \subsection{Subsection} \placeformula \startformula a^2+b^2=c^2 \stopformula \stoptext