I also just had this happen randomly in top orthographic view in 4.26.1, while the front and left views were fine.
Same solution: I selected a geo actor, pressed F to focus on it, and the grid magically reappeared.
I don’t know what sequence of actions I did to make the grid disappear in the first place in only one of the three ortho views active in the viewport, but it definitely seems to be a persisting bug.