It isn't too relevant really because an older poison dot is about the same efficiency as a newer nuke. So it's not like it makes much sense to use it. I guess at VOTS+ebolt levels it is more efficient to stack VOTS than use blizzard blast. But that's pretty niche regardless.
When you tested this on blue how did you do it? I cast the higher-level dot first, and the lower-level one stuck also when cast 2nd.
Relevant:
https://www.project1999.com/forums/s...ad.php?t=54537