Bad import of PDF text loses a double 'f'

Summary:

Somehow, importing the attached PDF results in a lost letter.

Steps to reproduce:

• download this pdf: plot_NNI_4_skinny.pdf
• import it in inkscape

What happened?

(full image)

What should have happened?

Version Info:

- Inkscape Version: 0.92.3 all the way till 1.0.2
- Operating System: Ubuntu 18.04 all the way till Debian sid at 2021-02-27

Original reported in this Ubuntu bug #1793881