I tried this locally in 9.0.5 in my app (relatively new, using atlas 3); and an integer field is just showing the text box.
You could try inspecting the drop-down with your browser’s dev tools. For me, the select exists, but has a style=”display: none” on it. It could be there is some custom styling (maybe on <select> elements) that forces it visible (e.g. with !important)?