Have a similar issue.
The transaction Id for one time purchase is not visible in the test user transactions.
Also when buying in the bundle (subscription + one time purchase), only the subscription is registered -confirmed by email confirmation and Roku Pay push notification.
Even though the purchase confirmation modal contains both.
It is impossible to test if payment works as it is not working for the billing testing channel...
What is weird, while purchasing one time purchase as a stand alone purchase, it is working (but still no transaction id associated for test user).