There is an inconsistency between two diagram-rendering paths:

• Saving a project renders diagram bitmaps successfully (used for thumbnails / embedded preview).
• Exporting a diagram to a file fails.

Both paths ultimately render the same diagram, so a working save proves the rendering pipeline is reachable. The export path must be diverging somewhere (different canvas, different target surface, missing parent, different DPI/scaling, output stream handling, ...).

Action:

• Compare the two code paths side by side and identify where they diverge.
• Establish whether the export failure is silent or raises; capture the failure mode.
• Fix the export path so it produces the same bitmap as the save path (or document why it intentionally differs).