SOLVED: Not a Brave bug.
In an existing Google Docs document, the toolbar appears when loading the page, but while scrolling down to the second page, the toolbar disappears.
Brave v 1.26.67, macOS Big Sur 11.0.1
No problem on Chrome 91.0.4472.114
Failure with shields up and with shields down.
Suggested chrome solutions don’t work. (e.g. Escape, Command+Shift+F …)