Example of a PDF with issues:
https://digibug.ugr.es/bitstream/handle/10481/47611/26750600.pdf?sequence=6&isAllowed=y

From what I can understand, this is not a poppler or okular issue, but seems to
be related to the fonts that are being loaded by poppler/okular. 

The document works fine at openSUSE Leap 15.3, as you can see at the screenshot
opensuse153.png, that also shows the dialog with the fonts used by the
document.

However at openSUSE Leap 15.4, some characters are missing (they show as
rectangles), as you can see at the screenshot opensuse154.png, which clearly
shows that a different set of system fonts is being used.

In case it helps, I could reproduce the same issue at Debian. The document
works fine at  Debian10 (debian10.png), but doesn't work on Debian11/Testing
(debiantesting.png). I could "fix" the problem on Debian Testing
(debiantesting-fixed.png) by removing the fonts that okular was using by
default (/usr/share/fonts/opentype/urw-base35/, /usr/share/fonts/X11/Type1/,
/usr/share/fonts/type1/gsfonts/), until it loaded Liberation2 in all cases, as
it was doing by default for Debian10.

This bug could be similar (maybe the same?) as
https://bugzilla.opensuse.org/show_bug.cgi?id=1192774.