[Zubersoft]

That is not a bug - it's most likely just related to the optimizations surrounding freeform annotations. If you go into the annotations editor, tap the settings icon and uncheck "Combine freeform annotations", then every stroke will be treated as a separate annotation. This is much less efficient, but if you need that level of control, then you can work that way. If you use the eraser tool, it shouldn't be removing anything outside of what you dragged over (assuming you have set the eraser to the drag over mode).

Mike