We've had a persistent bug up to SW2018 (havent tested with 2019 as yet) as follows:
Start a sketch
insert a sketch picture and drag the handle to reduce the image size leaving the 'scale tool' enabled but don't use it
Exit the sketch.
Now even though the sketch is small the zoom extents go to the size where the scale-tool is visible.
What's odd is that if you then exit the sketch and delete the sketch picture from the sketch it retains the zoom level. I cant find a way to re-set the zoom to the bounds of the actual sketch entities once this has happened.
Can anyone help?