Why isn't my custom Form API #states applied in my webform yaml editor working?
For the simple case where Question 2 is shown only if Question 1 is left unchecked, your YAML doesn't look right to me. If you're editing YAML in the conditions tab for the Question 2 checkbox, then just the last 3 lines should suffice:

invisible:
  ':input[name="some_question[q1]"]':
    checked: true


I don't think you want the "#states" key to be in there anywhere, nor do you want to include any selector information for Question 2.

That's if the YAML is part of Question 2's Condition source. If on the other hand the YAML you're showing is part of the form's source, then it should probably look something like this:

q2:
  '#type': checkbox
  '#title': 'Question 2'
  '#states':
    invisible:
      ':input[name="some_question[q1]"]':
        checked: true


Note that the #states key is shown here, but only once - we're omitting 2 out of 3 of the same lines as above.