Deregister iMessage not working on my iPhone 16 Pro

I have two phone lines (one work, one personal). Previously both were iPhones. I moved the number from one of those phones to a Google Pixel (new work phone). Before doing so, I turned off iMessage and Facetime. I also deregistered the number from iMessage on Apple's website. I received a confirmation that the process had worked.

Now texting between the two phones only works when I disable iMessage on the other iPhone. I have tried disabling and deregistering iMessage on the personal phone, restarting and re-enabling iMessage.

Every time I try to send a message from the iPhone to the Pixel, it attempts to send it as an iMessage and, of course, I don't receive it on the Pixel. This is a big problem as it suggests that every iPhone user with whom I have texted may have the same experience.

[Re-Titled by Moderator]

Original Title: Deregister iMessage not working