I have noticed the same problem when the browser is first started and I begin to enter text into the address field. Apparently, initialization of the page is very slow and hasn't been completed when the address field first appears, allowing text to be entered into it. A few seconds later, the rest of the page display is completed and, then, any text previously entered into the address field is erased.
I am using browser Version 80.0.3765.150 (Official Build) (64-bit) on Windows 10 Pro, Version 1909, OS Build 18363.720.