Thanks! Replacing "top" and "bottom" with "column:top" and "column:bottom" didn't make the MWE work, but once you pointed out that the locations of the marks to be fetched had to be column-specific, I started looking through the mailing list and found something that worked. I just had to change the \fetchmark calls in my MWE to \getmarking[TestMark][1][top] and \getmarking[TestMark][2][bottom].

On Tue, Apr 28, 2020 at 10:32 AM Hans Hagen <j.hagen@xs4all.nl> wrote:
On 4/28/2020 2:09 PM, Joey McCollum wrote:

> Is there some way I can improve my syntax to avoid this issue, or is it
> just a bug that needs to be fixed? I've tried fetching other marks
> (e.g., first and last), but the only marks that ConTeXt seems to find
> are the ones for the left column.
column:top etc

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