Contents of filled fields not being shown

Xreader 2.6.4
Linux Mint 20.1 Mate -but also on other distros, like Manjaro xfce, it's last 5 upgrades-.
And also with other document viewers and/or editors (okular - qpdfview).

Formulario G v3.pdf
Formulario G v3.pdf
Issue
The fields of a .pdf document filled with Xreader (or Okular, by the way) in my Linux machine, will appear to remain empty when you leave the field's "edition mode" (sorry, I don't have the vocabulary to describe this better).
Only when you return to "edition" mode , by double clicking on the field, will it's contents re-appear (show themselves).

Note: when you fill the fields with Adobe Acrobat Reader, the contents of the fields are shown even without going to the field's edition mode.

Steps to reproduce
I am attaching a file to show the differences:
1.- the "Juzg." field, was filled using Okular (that shares the problem with Xreader).
2.- the "Secret." field, was filled with Xreader
3.- the "M.P." field, was filled with Adobe Acrobat REader -on a Windows machine-
If you open the document with Xreader, you will see that "Juzg." and "Secret." appear to be empty, but the content of the "M.P." field is shown perfectly well (the last is the expected behavior, as I understand it).

Expected behaviour
The fields contents -after being filled- should be shown without the need to go into "field-edition-mode" (I don't have the propper bocabulary to write this..., sorry).

Other information
Similar .pdf files can be found in these web page: https://sir-aut.cba.gov.ar/formulariosWeb.aspx

Start by opening the attached Formulario G v3.pdf files in Xreader and comparing the Juzg., Secret., and M.P. fields. Investigate the document-viewing path responsible for PDF form rendering; done means values entered by Xreader or Okular remain visible after leaving field-edit mode, matching the Adobe Reader behavior.