Here’s my detective guess since you said it worked for some towers but not all of them and you got some extra sigils:
We know that before 3.3, there was an issue where Valravens could replace the raid boss. The devs told us they fixed that, and it’s kind of a funny story.
We know that the fix was the underlying cause of “Valravens don’t appear in Invasions” and/or “Valravens don’t drop sigils in Invasions”, because that’s how we got the story.
We know that the fix involved, “Valravens won’t appear in the 4-tower fight, you will just automatically get the sigils.”
So my guess is the code to generate a Valraven has several disconnected phases. What’s happening is the code is removing the “Boss” aspect of a Tower, but other parts of the code are recognizing “Oh, I can’t replace a tower”. So you are fighting a non-Boss tower and the ability doesn’t apply.
See A Couple of Invasion Issues for information about the bug, the hasty fix that caused more bugs, and the hasty fix that probably caused this bug.