On 2011-08-05, Wolfgang wrote with regard to section numbering not 
appearing in float captions when numbering is off for the section but 
called for by the chapter 
(http://www.mail-archive.com/ntg-context%40ntg.nl/msg57811.html):

    For the moment replace "number=no" with "incrementnumber=no".


and later that day 
(http://www.mail-archive.com/ntg-context%40ntg.nl/msg57814.html):

    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

This bug appears to still be with us. Has a solution since been 
proffered? If not, is a resolution in sight?

-- 
Rik Kabel