[NTG-context] How to track down source of [entry not flushed] displaying in indexes?

[Joel via ntg-context <ntg-context@ntg.nl>]

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I have some custom registers that I use to index some information. When compiled, it just prints [entry not flushed] instead of printing some of the page numbers.
I have tens of thousands of lines of code, so have no idea how to break the problem down to a minimal example. What does [entry not flushed] mean? What kind of feature in my document do I need to search for that might explain why this is appearing?

Here is the minimal code used in my index, as you see it isn't showing the error, so something else in the document is causing it:

 \setvalue{W3}{this is text for W3}


\defineregister[learnedshort][compress=yes]
\setupregister[learnedshort][style=sansbold, textstyle=slanted, n=3, pagenumber=yes, indicator=no]
%
\defineregister[learnedlong][compress=yes]
\setupregister[learnedlong][style=sansbold, textstyle=slanted, n=2, pagenumber=yes, indicator=no]
%
\define[1]\learnedis{%
    \txt{} \getvalue{#1}
    \learnedshort{#1}
    \learnedlong{#1}
}
\define[1]\learned{%
    \textreference[textref#1]{\expanded{\currentstructuretitle}}
    \reference[ref#1]{}%
%
    \learnedshort{#1}
    \learnedlong{\getvalue{#1}}
}

\starttext

\learned{W3}

        \startchapter[title=Standards]
              \placeregister[learnedlong]
        \stopchapter
\stoptext



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