When you have an empty folder and you click the right-facing arrow on the file tree, the right-facing arrow disappears. If there is content in the folder, the right-facing arrow turns into a down-facing arrow. This makes it so there is no visual cue confirming that you have opened an empty file.
Since I expected the arrow to turn into a down arrow, I knew that some unexpected behavior had occurred… and I reasoned wrongly (since I thought the contents I was viewing were already in the empty file!) that the listed files were already being shown despite being in the folder, rather than that the down arrow was failing to be shown when it was clicked.
I know it’s silly, but if there is ever a time when you need a visual cue that a folder has been opened when it was previously closed, it’s exactly when the folder is empty. If you have files in the folder, it should be easy to note the change when you toggle it from open to close.