The space after the register entry is only ignored when you put it
before the start of a paragraph because ConTeXt saves the content
and flushes it at the start of the next paragraph.

When the register entry appear within in a paragraph or when TeX
is in horizontal mode (like in your footnote) the space after the argument
stays and appears in the text. In this case you have to put a comment
after it to ignore the space which is generated from the line break in your
document.

Wolfgang


Rik Kabel schrieb am 27.11.18 um 21:17:

And now I can construct it, so here is a not-necessarily-minimal example. Note that the problem occurs in footnotes (and endnotes, left as an exercise to the curious).

\setuppapersize[A9]
\starttext
\startparagraph
Text
\startfootnote
Text
\stopfootnote
\stopparagraph
\startparagraph
\index{Strumpf}
Text
\startfootnote
\index{Strumpf}
Text
\stopfootnote
\stopparagraph\startparagraph
Text
\startfootnote
\index{Strumpf}%
Text
\stopfootnote
\stopparagraph
\stoptext

With the result:

What is unexpected is that the space does not appear in the body (non-footnote) text of the second paragraph. The difference between handling in notes and in body text seems discordant.

--
Rik