I have a similar issue with PhotoLab 4. When ever I open PL4, the image browser pane is minimized and it takes many, many attempts of clicking and dragging until it finally appears. Some times it works the first try but most times can take a half dozen attempts or more until the image browser pane actually pops up. Also, doesn’t matter if in customize or browser mode.
Very annoying.
MacBook Air M1, Big Sur
ps. The issue also occurred in Monterey beta as well, FWIW.