Data validation bug?

I have a simple data validation formula to check whether a valid number is entered or not. It works, provided that the user types the number, then presses either the ENTER or TAB key.

If they type a number, then click another cell using the mouse, the number will be accepted, with apparently no data validation check kicking in. In other words, it will accept an invalid number.

I have style STOP selected, I have IGNORE BLANK clicked on (though I don't think that even applies here).

Am I missing something really obvious here?