When the ulti ends, his spirit releases the illuminate instantly before it reaches full channel even when the spell was casted before the ult ends.
That's quite annoying because it means that you can't really cast illuminate properly in the last few seconds of your ulti, you have to wait till it ends even if you see a good opportunity.
It also means you have to constantly watch the ult timer (and when it comes down to the last 10 seconds or so it's actually really hard to tell whether you have 3 seconds left or 6 seconds left in the ult) so that you don't accidentally cancel your own illuminate when the ult ends.
Simple fix: just let the spirit channel illuminate fully if casted before the ult ends, even if ult ends during the illuminate channel. There is no scenario where this would be even remotely imba or OP