Drawing on OneNote for IOs

Apple Pencil double-tap eraser flips to a text pen, and drawing pens randomly grey out. Support has not fixed it.

Honestly, both of these smell like app side bugs rather than anything you are doing wrong, and the fact that support keeps recycling the same steps every week says they have not got a real fix either. That said, here are the things actually worth checking, because they fix it for some people.

For the double tap flipping to the text pen, check two places. In iOS Settings, Apple Pencil, set the double tap action to "Switch between current tool and eraser". Then in OneNote itself look in the Draw settings for the Apple Pencil or "double tap" behavior, because OneNote can override the system setting, and the "text to type" behavior you are describing sounds like the ink to text pen is getting selected. Pin the eraser and the pen you want to the Draw ribbon so a misfire is easy to undo.

For the pens greying out, that usually happens when the page is not fully in an editable state. Check that the section is not password protected, that the notebook is finished syncing rather than sitting offline (a half synced page goes read only and greys the ink tools), and that your account is not silently in a "needs sign in" state. When it greys out, try switching to another page and back, or toggle Draw with Touch, which often wakes the tools up without the long wait.

If it keeps happening after all that, reinstall is worth one shot, and please file it in Help, Feedback inside the app, since pen regressions on iOS get fixed in app updates and that is the channel that actually moves them. I know that is not the clean answer you wanted, but you are not imagining it.