The last time I met with Rick Welch, he noticed I was doing that, except in my case it was my cheek rather than my nose that was bleeding. He had me switch on the spot from a tab I was using to a glove, and that seems to have taken care of it. Rick said the same thing happened to him, which is why he uses a glove rather than a tab.
The string passes very close to your nose, regardless of what you do, and a very small change can change a near miss to a near hit. Even now that I've switched to a glove, I still hit my cheek or nose on occasion, without really knowing why I'm doing it. As with you, there seems to be one bow it happens with more than others. I suspect I skim my nose from time to time, but as long as it doesn't happen enough to cause it to bleed, it isn't a problem. When it does cause it to bleed, then every little contact will start it bleeding again, and I have to make a conscious effort to rotate my head further out of the way than I usually do until it has a chance to heal. Maybe I should be doing this anyway, as I've read that the more you rotate your head to face the target, the better off you are.
It seems to help, and I think is probably better for my form, if I don't lock into my anchor so much, but instead make a more delicate contact with my face. I think when I lock into my anchor I may have a tendency to rest on it, whereas when I barely touch my face I have to focus more on maintaining my back tension. I don't really like anchoring this way, because I'm not used to it, so it is still more of a thought rather than a recommendation at this point.