Can't create multi-line comment

According to the feature announcement, I should be able to enter a comment or code suggestion that applies to multiple lines of a PR by dragging the blue plus icon down. A blue line appears showing the extent of the selection.

But when I do that, I don’t get a blue line, I get a second plus sign that moves with my mouse pointer. Any comment I create applies only to a single line of code, which means I can’t suggest edits to multiple lines of code.

This post suggests I can alternately select a block of lines by clicking one line number and shift-clicking a second. But that doesn’t work either; shift-clicking a second line simply deselects the first.

Are you looking at a commit that is part of a PR diff? Or an existing commit to the repository? For example if I go to a repository’s Code page --> commits, and then pull up an existing commit I see what you see. However if I go from the repository’s Pull Requests page, open a PR, then open a commit within that PR I can select multiple lines to make a comment on.

This is a PR diff. Same behavior whether I’m using the side-by-side or the unified view.

I used to be able to do this, and now I can’t either – any idea why? I really enjoyed that feature.