I finally traced the problem after a month of from time to time guessing I was gone mad, since I couldn't reproduce it.
I don't have a screenshot, but I can attach one if needed, tho I'm sure I can explain the problem now.
Following the few step you should have the problem:
- create a chart in a shell of dimension 1/5 of your screen (1/5 is just to better notice the problem)
- resize the shell to fullscreen
Now try to get a tooltip. It should be shifted along the Y-axis.
Sorry for the delay, I noticed it by chance.