The following example produces the expected result under --luatex. It hyphenates the long word and positions the chapter number within the frame. Under LMTX, the long word is not hyphenated and the chapter number is shifted out of the frame.

    \setuphead [chapter][alternative=margintext]
    \starttext
      \chapter {Establishment}
        \input klein
        \contextversion
    \stoptext

The problem goes back at least to version 20240621 (running on https://www.context-on-web.eu/).

I did not experiment with other mechanisms that place text into the margin.

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Rik