When saving and closing QGIS with several open attribute tables, QGIS does not open these when opening the project next time. This is very annoying. Is there a workaround for this?
No, attribute table state (open or closed) cannot be saved. The one work around I can think of is to use the Project menu>Project Properties openProject() python method to loop through the canvas layers and open them that way for a saved project. See past Q/A of how to open an attribute table by code: