There are multiple reasons why that could happen, such as some webhook failing. However, if the more trivial reasons are not the correct one, that's something I have observed (that might also affect you):

I had a similar situation where I observed the following: my invoice was supposed to be drafted on July 24, 7:22AM. However, I decided to advance with test clock up to July 25, 9:42AM so I would have expected that the invoice was already finalized.

Instead I saw the invoice as draft and Stripe was saying: "Subscription invoice will be finalized and charged 7/25/25, 10:42 AM", so 1 hour after my test clock.

I then tried again with a different subscription by advancing to the exact moment in which the invoice would be drafted (rather than later on) and, in that case, I still saw the message saying that it would be finalized 1 hour after.

In both cases, the invoice was actually finalized by advancing with test clock by an additional hour.

So I think it might be a kind of bug on Stripe side for which the invoice is not finalized at the correct moment but it instead depends on the way you advance with test clock. Can you check if it's the same for you?