Hi Marco,

It's a bug, I think.
The keys that control this are way and prefixsegments:
\setupformulas[way=bychapter, prefixsegments=chapter]

way=bychapter means 'resume numbering at each chapter'
prefixsegments=chapter means 'precede each formula number with the
chapter number': chapter.formula

Right now, as soon as I set 'prefixsegments' to 'section' or 'chapter'
or a number, the prefix segments disappear. Ditto when I set 'way' to
anything. This suggests that it is a bug. Is it indeed, Hans? I've
attached a test file for you.

You can use \setupformulas[prefixsegments=] to solve your problem if
you never want prefixes; that also specifies the correct behaviour
once the bug is fixed.

Cheers,
Sietse