Exporting PDF with transparent lines (highlighter) won't have a uniform opacity

[Semphriss]

Reproduction steps

1. Open any PDF file.
2. Draw a single continuous stroke with the highlighter tool, preferably passing several times over the same place.
3. Export that document with the "Share" feature.
4. View the document.

Expected

The highlighter stroke should have a uniform opacity.

Actual

The highlighter stroke is much more opaque in areas where the stroke was drawn slowly or where it passed several times over itself.

Additional notes

What's causing this is the drawing of each segment of a stroke individually, rather than at once. I should check if pdf-lib supports a canvas-like continuous stroke mechanism.