Figure export issue when sub-figures are present

### Bug
When a figure in a research paper contains sub-figures, sometimes the sub-figures get split into multiple PNG files as opposed to a single PNG file. This doesn't always happen but I encountered the issue multiple times.
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### Steps to reproduce
I used the example code from [here](https://github.com/docling-project/docling/blob/main/docs/examples/export_figures.py) on the attached PDF. Specifically, Figure 2 in the PDF contains 4 sub-figures and it resulted in 4 PNG files.
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### Docling version
Docling version: 2.30.0
Docling Core version: 2.27.0
Docling IBM Models version: 3.4.1
Docling Parse version: 4.0.1
Python: cpython-310 (3.10.16)
Platform: Windows-10-10.0.26100-SP0
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### Python version
Python 3.10.16

[NEJMoa2201445.pdf](https://github.com/user-attachments/files/19858469/NEJMoa2201445.pdf)

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