If you have two panels open (e.g., a Whiteboard on the left and Rich Text view on the right), then open a third panel on the right, when you close the right panel the original Whiteboard panel re-appears on the left, but it’s empty.
Moving to another view in the Space or another Space, then returning to the Whiteboard restores the view. Seeing this on both the web app (Chrome 131.0.6778.265) and desktop app (2.1.0 for macOS).
I have a screen recording, but as a New User I cannot currently upload.