I have resolved the issue in a practical, but unsatisfactory, manner.
I have resorted to creating an interlude, a new heading derived from
title, setting it for left pages with no displayed head:
\definehead[ChapterEpigraph][title]
\setuphead [ChapterEpigraph][
As a followup to my query on suppressing header marking, I have prepared
an example which clearly shows odd, if not buggy behavior.
The book places chapter openings on recto pages which follow a verso
that may have an epigraph. When there is no epigraph, the blank verso is
properly unmarked by
