Interesting. I'm seeing this too:

I believe I noticed that the "Enter key" functionality depends on the
presence of an in the form. I had a form where I
replaced it with a (to allow underlined letters) and the Enter key
stopped working there and then.

I was actually Googling up how to stop folks from being able to submit a form by pressing return in a textbox, but it looks like if you remove the Submit button (and/or turn it into a normal type="button" INPUT element), that already happens.

Interesting. Hope that's a crossbrowser solution. Hello, testing time.