Hi,
When the screen geometry changes across sessions, the PixInsight Core application performs a reset of tool bars and other GUI elements for sanity. This is done because, for example, some tool bars may become inaccessible if the screen geometry is reduced significantly.
In your case, I agree that this behavior is a pain if you have to use the application alternatively with and without a second monitor. I'll try to improve on this for the final 1.8.0 release, i.e. by implementing a smarter and/or user-selectable automatic tool bar reset feature.
Sorry for the inconvenience.