Ah! But I’m using a Magic Keyboard 2 and the numbers on the main keypad, not the numeric keypad.
The odd thing is that, if I don’t press the shift key, I get the behaviour that I described but, if I follow strictly the hint on the popup menu for labels and only press the ^⌥ combination instead of ^⌥⇧, it would appear that those keystrokes are “sort of” valid(ish), except when they are not.
I also use a Magic Keyboard and I have the same behavior than @Joanna. As I find it is more convenient to use the numeric keypad, I had never experienced this problem. But this evening I made the test on a MacBook Pro… without the numeric keypad. And is is very anoying to use.