Elder Guardian Not Transitioning

You guys probably already know about the boss transition problems at this point, but I thought I'd throw this one up just in case. It appears that when I'm running delirium and kill the boss while it is frozen due to the delirium effect it doesn't phase. Maven witness might be a factor as well thought I'm not sure about that.

Here's a video showing the bug:
https://youtu.be/X4MLFc9kyIk

Bug report reference number: 3,244,141,216