حُفِظَتِ النُّسَخُ الْقَدِيمَةُ.
The old copies were safeguarded.
حَفِظَ الْأَمِينُ النُّسَخَ الْقَدِيمَةَ.
The custodian safeguarded the old copies.
In the first, “ḥufiẓat” is a passive past verb and “al-nusakhu” its deputy subject. The wording directs attention to what was protected. The active counterpart names the responsible agent and is suited to a discussion of accountability.
A passive construction does not necessarily imply an unknown agent. The agent can be known yet irrelevant to the current purpose. Both expressions may be grammatically correct while giving prominence to different information.