maybe a "position lock" button could be added. would need investigation.
that could additionally strip the scrollbars when activated (as a wild guess) whilst locking the current "location" inside any particular zoom level. As I say, regardless of the mechanism, it's a "new" facility.
Thus feature request
but yeah I suppose you could class the other thing as a bug.
A further look at the code might reveal why the other thing occurs.
Probably running rgbpaint locally on a non-kindle machine could give us a base for "intended operation". Perhaps it is just, oddly, coded that way.