Manage Display formatting settings won't work
Manage Display is for the front end, not the back end (i.e. not the entity edit form).

When the field value renders on the front end, you'll see the formatting you've chosen.

There's probably not much point trying to force formatting for a number in the backend, it makes more sense to take the raw number and then only manipulate it upon output. But if you absolutely need the functionality, it'll likely be a custom JS job as Drupal doesn't have anything built in to handle it.