I have to admit, I don’t have the most up to date iPadOS on my iPad 12.9". If something like this happens, I usually change orientation of the iPad (from portrait to landscape and back to portrait). This brings back full display.
Well, yes I am now on iPadOS 26.3 but the issue remains.
However, I discovered that there is a “Show toolbar” command in the Jump bar that does restore the spontaneously disappearing toolbar, although its key command (Command 6) has no effect.
It would appear that the window dimensions are spontaneously altering.
I am using the “traditional” iPad full screen / single app display rather than multiple windows.