Why taxonomy term view is showing only a single content?
After following @ShawnConn's advice on enabling the Show the SQL query option, I figured out what happened.
The query was indeed returning only one result. But then I remembered the categories were recently changed right on the database. To do that, we've updated the respective field table -- node__field_categories in our case. However, the taxonomy_index table was not updated. I went ahead and manually (again, I know) updated the taxonomy_index table and now the view is showing the expected content. I hope this is the last and only side effect.
The lesson learned is: only touch the database if you really know what you're doing. And even then, think twice. :P