About 90% of the time, when I type 6I (it works correctly in this setting) it autocorrects to 6i. Even if I paste 6I it will still correct it to 6i. If I type a 6, a space, then an I, and then remove the space, it corrects to 6i. It doesn't do this with any other number combination. It happens in Spotlight, passwords, Safari search, and other cases. But it works just fine in MS Word or here. I've disabled autocorrect, shut down any optional apps, changed all my keyboard settings to do nothing extra.