The filter function in the Excel web app simply doesn't work. Here I'm choosing to show all statuses except Done, and it shows it anyway:
There actually are some rows with a status of Done that are hidden, but I can't see any pattern to it. There's nothing special about this row, as far as I can see.
Best Answer
I figured it out, almost by accident. You see how in the original screenshot, the row colors alternate but then stop at row 23. That's because the "table" part of the worksheet only included the top rows, and the filtering only works on those rows. I had to reformat ALL of my data as a table, and now it works. This is really non-obvious.