This most probably happens because normally tooltip appears besides the mouse pointer, but if it doesn't manage to do that (since it is too large) the window loses focus to the popup and then closes its tooltip re-gets it and opens it again.

The attached example application can be used to reproduce the issue.

Steps to reproduce:
1. run the example
2. move mouse cursor on top of the application window

Expected result:
A large tooltip is shown

Actula result:
The tooltip shown is flashing