Strange project window pane split upon startup

Project windows restored upon startup always appear with the left sidebar taking up a majority of the widow's width, with the remaining panes squeezed on the right.

Oddly , if multiple windows get restored, only one exhibits this behavior (i.e. one window restored has a strange split, but if three windows get restored, only one of them is affected). So far, I haven't been able to determine what determines which window gets affected in such group.