[palosanto]

Mike, Javier
I haven't been following very closely the problem with the text annotations because I don't really use text very often.
But I was trying it earlier and I thought  would share my findings. When I type a text annotation a couple of odd things happen when zoomed in:

This seems to apply to both of my devices as far as I can tell:

When I'm zoomed out 100%:

The first tap brings the keyboard up and shows the cursor ready to type, exactly where I tapped.
If necessary the screen will shift up to accommodate the keyboard if the chosen location is low enough to require it so that the keyboard doesn't cover it. This is normal.
So everything works as it should. 

While zoomed in:

What seems to happen: The first tap displays the keyboard and the cursor along with some recent text annotations seem to either shift up a lot or disappear altogether and I can't see what I'm typing. Then when I tap on the screen to finish, the keyboard disappears and the new text shows up along with the other annotations that had disappeared.

What actually happens is that the cursor and recent text annotations have moved up either off screen or up on the screen along with the whole score shift trying to accommodate the keyboard and they are not obvious to see as they are so far away.
I think that MSP calculations for shifting the screen up to accommodate the keyboard are not correct when zoomed in so the screen moves up even when it wouldn't need to when I'm choosing a text location high enough in the screen to require a shift.  So it ends up off screen or way higher that expected.

Mike, I have the feeling that you are scaling also the restricted area covered by the keyboard so when you are zoomed in and type on a location that wouldn't normally be covered, it thinks that is bigger and causes the shift even though it's not really needed because the keyboard would have fit perfectly. 
And the shift seems to increase the more you are zoomed in.

I hope I'm making sense ...