Hello @JosephRhodes2 , I assume you are using ArcGIS Online, but if you are using ArcGIS Enterprise please let us know which version it is.
I enabled the Fullscreen tool of my Map Widget, and it works as I expected. Pressing Fullscreen (showing as a maximise icon) went to fullscreen, and could revert to minimised using either the ESC key, or the Fullscreen button again (now showing as a minimise icon).
What browser are you using, I tried it in Chrome & Edge ?
Can you post a sample Experience that we can try ourselves ?