More info:
The problem is somewhat intermittent: Occasionally the script works as expected, and sometimes changegrep misses the last two or even three matching paragraphs.
This made me wonder if there’s a problem with asynchronous execution, such that the changegrep commands are running before the earlier commands are finished building the text frame.
I moved the code snippet to the bottom of the script to execute after a couple of other things that take two or three seconds. This appeared to help–changegrep worked more often–but it’s not a “solution” because I need certain things to happen before others.
I put the snippet back where it belongs, and inserted a “fit frame to content” command just before it:
textframe.fit(FitOptions.FRAME_TO_CONTENT);
This seems to have solved the issue … perhaps “fit” waits for InDesign to finish building the text frame before executing and allowing the script to continue.