Sometimes, the widget does not reappear after reboots. This does not happen every time. I would say about half the time. No changes to permissions or settings.
Also when the app is placed in the list of sleeping apps, the widget disappears from the home screen. I am on the Galaxy S10+.
I’ve seen similar behavior when a widget depends on data that isn’t available yet after a refresh or navigation event. It might be worth checking the widget’s visibility conditions and any data bindings to make sure they’re still valid when the screen reloads. Sometimes adding a refresh or rebind step fixes these disappearing widget issues.