[NTG-context] How to fix indexes displaying [entry not flushed] with hidden data?

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

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I have a document that needs to place some "hidden" data. In other words, there is an entire \input file with an article, but it doesn't get rendered visibly. However, the indexes, registers, and bibliography need to still function as if it were displayed right there on the page.
It should be as if the data actually is all there, on a single page, but either compressed inside a single invisible pixel off or plain invisible to readers, not using any physical space on the page.

I tried two methods suggested at Stack Exchange for hiding the data:
1. \startnointerference
2. \setbox\scratchbox=\vbox{}
These both successfully seem to place the data there, without it actually being visible and bibliograhy entries still function fine.

The problem is, index and customer registers are getting confused:
When one goes to render the page numbers in \placeindex, it gets confused, displaying the message: "[entry not flushed]" instead of the page number. When I render this minimum working example below, the index is populated with [entry not flushed] instead of page numbers. Note that if I delete the test "This is invisible text."and recompile, I can get a successful render. The problem is, I have text that needs to remain invisible.

\starttext

    \index{birds}
    \index{insects}


        \startnointerference
            \index{spiders}
            \index{bears}
            \index{rabbits}
            This is invisible text.
        \stopnointerference
    
    \input knuth
    
    \placeindex
        
\stoptext


How to fix indexes displaying [entry not flushed] with hidden data?
--Joel


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