To replicate, simply run the the Android native sample app and replace code...
with your own...
The onResult of the manualCheckoutSession fires once payment is complete or cancelled or whatever, but the onReady never fires.
I expect onReady to fire after calling checkoutButton.init(.....) to let me know that the button is ready.
Am I misunderstanding something?
I will take a look and get back to you soon.
Please email this inquiry to firstname.lastname@example.org, and someone will connect you to the Visa Checkout team. The VCO team will then provide you with an answer to your question. Enjoy the rest of your day!