Roll out an update that defeats the end to end encryption in some subtle way that wouldn't go noticed for a few days. They'd be told when to do it for maximum effect, and if the window is small enough it might even go unnoticed for far longer when another uncompromised update overwrites it. They have no duty to report such things to relevant authorities even if it was discovered internally, so you could be looking at some corporate coverup that while not in on it, seeks to minimize liability/embarrassment.

Really, can you possibly tell if your Signal messages were compromised? Now that iPhones aren't really jailbreakable, you can't even see inside your own device.