When opening an Office file stored in my OneDrive for Business by clicking on it in Timeline, the file does not open in the corresponding web or desktop Office Application.
Instead, my browser downloads a read-only copy of the file (and presents its usual file download dialog: "Do you want save or open this file.")
(It doesn't matter which browser is used.)
To reiterate, the problem is that Timeline downloads a copy of the file instead of opening it.
It didn't always do this, but I can't say when the problem started.
I am on Win 10 Pro, Version 1909, Build 18363.535