What version of Blesta are you running? This was changed in 4.1, see https://dev.blesta.com/browse/CORE-2284
If you are running 4.1+ try testing with the Wizard List order form, I've personally seen the out of stock button appear on this order form. If it's an issue with wizard boxes we can look into it. If you're not running 4.3.2, you should be.