I noticed that a 5x5px icon in my file was exporting as 6x6px. After some experimentation, I determined that the problem was that the layer I was exporting was no aligned to the pixel grid (i.e. it’s position was a non-integer). When I aligned it to the pixel grid, it exported at the correct size. This naturally makes no sense; a layer’s position in the file, or the position of it’s parent layers, should have no impact on the size of that layer when it is individually exported.
I found other threads referencing this issue (as far back as 2021), and a member of the Community Support team posted that the issue had been fixed in Little Big Update #13–however, that update only seems to fix a similar (but unrelated) issue regarding exporting assets at multiples of the original size.