Should I always force the use of active voice in my dissertation?
The goal of style guides is to give suggestions for how to write well. These are guidelines and suggestions, not immutable rules. Once you understand what the base suggestions are, and why they’re there, you can begin to understand when not to apply them. As you said, if you don’t know who the “actor” is, an active-voice sentence becomes more difficult to make clear without sounding stilted.

(I’ve tried writing a paper entirely in active voice when I was still a student. I wasn’t particularly thrilled with the results.)