Well, obviously her visions only show her the future as it would have been, had she not received that vision, or something like that. But since she can see counter-factual futures the fact that her reaction would undo a particular future can't explain why she doesn't see that future in the first place.
You could further stipulate that she can only see futures that are indiscriminate to the way her visions counter-factually ended up not to containing them , i. e. that turn out the same whether she counter-factually changed her mind on trying to see that future or counter-factually tried but failed, or something similar (e. g. that she counter-factually received some other vision, counter-factually based on yet another, but only when the visions cycle between a limited number of possibilities, which leads to similar results with somewhat different details).
This would not explain why her visions fail even if the undecided subject is sufficiently far away not to be influenced by nuances of her counter-factual reactions, i. e. when trying to see what someone is doing between now and time X when X is the earliest possible point when she could affect anything. Maybe she can't see anything she couldn't possibly see with her own eyes no matter what she does, so she can't receive those sorts of visions in the first place. But finding out would still be useful (e. g. so she could arrange for really fast transportation on hand to increase her vision range). And if what's blocking her visions vs. "undecided" things is of the sort speculated about above she could circumvent it with suitable precommitments and staying outside the subjects actual sensory range (while the subject is still within her potential sensory range).
As for your explanations why your Bella does not care to find out, it's your character, but it does seem somewhat at odds with the stated irresistibility of mysteries for her, or the way she insists on testing the various vampire abilities. Also finding out might have allowed for a more convenient way to block her from reading her notebook entries, and possibly allowed her to find James.
In the vein of the Harry Potter and the Methods of Rationality discussion threads this is the place to discuss anything relating to Alicorn's Twilight fanfic Luminosity. The fanfic is also archived on Alicorn's own website <strike>(warning: white text on black background)</strike>.
Previous discussion is hidden so deeply within the first Methods of Rationality thread that it's difficult to find even if you already know it exists.
Similar to how Eliezer's fanfic popularizes material from his sequences Alicorn is using the insights from her Luminosity sequence.
Spoilers for the fanfic itself as well as the original novels need and should not be hidden, but spoiler protection still applies for any other works of fiction, except for Harry Potter and the Methods of Rationality chapters more than a week old so we can freely discuss similarities and differences.
EDIT: Post-ginormous-spoiler discussion should go to the second thread. (If you have any doubt on whether you have reached the spoiler in question you have not.)