Extension calculates positions and draws highlightings precisely for the state of the content that was on the page at the time of clicking the 'Find' button.
After drawing the highlightings, the extension doesn't track changes to the content.
So it doesn't redraw the highlightings when changes to the content occur and the content itself redraws.

The extension could automatically rerun the search logic each time something happened to the page by listening to various page events. But there are pages where all this computation would be perceivable enough, in terms of time and resources taken, that you wouldn't want it to happen that often and uncontrollably.

An easier solution is to just remove or rerun the search manually exactly when you need it.

So I'm afraid from the algorithmic standpoint it works as good as it can.
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Well, I use Firefox 70 (64bit) myself, and I use this addon almost every day when I need to find something on the page. I think I would notice if it didn't work at all.

But of course, there is a chance of a bug in any script.
I could look into it if you told me the website where it happened and the exact steps that you did.