Scope
A changed "Next billing date" is not sent as an event when using the Advance Invoice feature. Why?
The subscription change event doesn’t occur while creating the Advance Invoice option.
Solution
A subscription change event happens when there is a change in subscription.
However, with an advance invoice, we simply generate the upcoming renewal in advance. So that doesn't come under subscription change because this is something that was about to happen but you simply did it prior.
Thus, the subscription change event doesn’t occur while creating the advance invoice.
However, when you are manually changing the billing date to another date, that means that you are interrupting the normal renewal flow and changing it to some other date, and that is why the "Change subscription event" occurs.
So if you want to get the "next billing at" for the subscription, then you can use the List Subscription API to fetch that.
Related Article:
Change next billing for multiple subscriptions.