Much like the now closed bug
https://github.com/bigbluebutton/bigbluebutton/issues/9348, but on 2.5.12
Basically, approximately the left 25% of the presentation does not allow drawing (the cursor changes from the red dot, to a normal cursor). If the Public Chat is hidden, the area reduces a bit (so it's maybe 20%), and if the user list is also toggled closed, it reduces again (to maybe 15%). Finally, if the presentation is made full screen, we get down to maybe 10%.
If the line is started outside of the area, and the mouse held, it will continue to draw into the area, but trying to start a new line within the area doesn't do anything.
This affects all tools (not just pencil).
Problem doesn't appear to exist on the BigBlueButton demo site, but that is 2.6 and I think there were some changes made to the whiteboard.