How do you support incrementing a `role="spinbutton"` with Voiceover?

I'm struggling to implement an accessible input with increment/decrement behaviors, using an <input type="text" role="spinbutton" /> element in HTML/JavaScript. But it seems like with VoiceOver there are custom (fake?) input events for incrementing/decrementing which change the input's value in unexpected ways.

How do I make sure a user navigating the page with assistive technology doesn't get bad guidance from VoiceOver while trying to interact with my widget?

For example, using super trimmed-down code like this:

function logIt(...args) {
  document.getElementById('output').appendChild(
    document.createTextNode(`${args.join(', ')}\n`)
  );
}

document.getElementById("myInput").addEventListener('input', (e) => {
  debugger;
  logIt(e.type, e.data, String(e));
  e.target.setAttribute('aria-valuenow', e.target.value);
}, false);

document.getElementById("myInput").addEventListener('keydown', (e) => {
  logIt(e.type, e.data, String(e));
}, false);

      <input
        id="myInput"
        type="text"
        role="spinbutton"
        autocomplete="off"
        defaultValue="1"
        aria-valuenow="1"
        aria-valuemin="0"
        aria-valuemax="100"
      />
    </div>
    <pre id="output"></pre>

... VoiceOver will describe the input as a "stepper" and give instructions on how to increment/decrement using the keyboard. But using those keyboard commands results in some weird math, as seen in this screen cap:

You can also see (from the "logging" in the screen cap) that when the user types input, an InputEvent is triggered with event.type being input - but when the VoiceOver keyboard command for increment/decrement is used, a base-type Event is triggered with event.type again set to input.

And this doesn't seem to be unique to my implementations of role="spinbutton". The jQuery UI spinner doesn't behave well when incremented/decremented using VoiceOver keyboard commands:

I even tried some of the w3c's examples for role="spinbutton":

• The font size widget on https://www.w3.org/TR/wai-aria-practices/examples/toolbar/toolbar.html
• The date picker on https://www.w3.org/TR/wai-aria-practices/examples/spinbutton/datepicker-spinbuttons.html

... and even though VoiceOver described each of those UI controls as a "stepper" and gave instructions on how to increment/decrement them using the keyboard, those instructions didn't seem to work. Other keyboard behaviors worked - but the ones VoiceOver suggests don't.

What can I do to make sure that role="spinbutton" markup works correctly with VoiceOver's increment/decrement keyboard commands?