The Add-to-cart button is the only way to send the filled form. If you press ENTER it actually presses the Add to Cart Button.
Just tested on https://demo.uwix.nl to enter a number and press the ENTER button. The required fields still popup. Also after using TAB to switch to the Add to Cart button and using SPACE and ENTER the result is the same.
It's not usefull to check every seperate field because using ENTER is still just sending the form or better said "it presses the Add to Cart button".
We'll do some more testing later this week, just to be sure.
Bug? That's not a bug, that's a feature. :-)