Since I can confirm a tooltip text generated by GSX that might be remained stuck on the screen (for example if you closed the GSX menu with the toolbar when the tooltip was still visible) is always 100% removed by opening/closing the menu again, and in addition to that, a GSX tooltip should always contain some text, the only possible explanation is some kind of simulator bug that not only prevent the text to show, but it's also affecting the command to remove the tooltip the menu surely sends.
All menu systems in the sim is done with Coherent GT, which at the very end interacts with DirectX (video drivers), so maybe this might be related to drivers or video settings.