Exporting visualization as PNG file doesn't save axis labels or any text

In the browser I see this:

When I click Export > Save as PNG and open the downloaded file I see this:

For reference: https://usegalaxy.eu/u/sbray/v/em-pot.

Start by reproducing the PNG export from the linked Galaxy visualization and compare it with the browser view. Trace the export path to identify why axis labels and other text are missing; done means the downloaded PNG preserves the visible labels and text.

Written by the indexing model from the issue text.