So I've entered Serana's mother's secret lab and have put the ingredients in the bowl, been soul trapped, and the portal to the soul cairn will not lower and open. It does drain health and the journal is fully updated so I know I've done all the steps. When I stand on the unopened portal it says "To Soul Cairn" but I can't go through because there isn't a way down, it's still just health svcking floor.

I've never heard of this bug before, interesting...

I can't help you, you'll have to wait until Bethesda patches this (if they even do it)

Do you mean for the first trip, the quest trip? Do you have Serana as a follower? I do, & had no problem with the quest & after reading complaints that people can't go back, I tried & had no trouble returning, finding mother & giving her the good news.

the first trip, this character has never been to the soul cairn.