Hi Joseph Aldcroft,
I'm sorry to hear this happened. I understand how frustrating it is to find that a license you paid for has apparently been removed after an update, especially when you no longer have the original product key. According to your description, this appears to be related to the recently introduced KB5002907 update scenario.
Microsoft has published information about this issue and the available resolution steps in the following support article: KB5002907: Optional update for out-of-date Microsoft 365 Apps installations.
I would recommend carefully reviewing that article and following the guidance provided there, as it contains the official instructions intended to address licensing and activation issues associated with this update.
I realize this situation is very concerning, particularly given the cost of your Office 2019 purchase. Hopefully the steps in the article will help restore normal activation without requiring the original product key.
Sorry again for the inconvenience this has caused, and thank you for your patience and understanding.