They have turned neutral in the past and they still do, but only briefly.

Several walkthroughs of Root Sunder on YouTube confirm that the mobs there turn neutral upon completing Forgotten Soul, including one as recently as June 13.

But I also did both Forgotten Soul and If the Dead Could Talk on my main many years ago (back in 2017), and I just stopped in on that character to confirm that the mobs were hostile to me again (they were). It's been that way for a little while now at least (see below), and I can only recall more recent trips when the enemies were hostile.

I then got a character who hadn't done either quest. The enemies were initially hostile, as expected. They remained that way after turning in If the Dead Could Talk and returning to the dungeon to work on Forgotten Soul. As soon as I handed in Forgotten Soul, the creatures were neutral, as in the videos above. There is a moment during that quest when the screen whites out, and after that the mobs got their yellow outlines and were no longer aggressive.



I then left and immediately returned to the dungeon, at which point the mobs were back to being aggressive.

The reversion to hostile was listed as a bug on UESP on January 27 of this year, if I understand it right, so someone had noticed it by then. But whether it was always there or when it was introduced is not clear. And it doesn't take a relog to make it happen - just exiting and returning results in the same thing.

The videos I've looked through so far don't show whether the mobs also turned hostile again re-entering the dungeon post-quest in the past. (Most walkthroughs wouldn't have a reason to return once things are done.) So it would take some more digging to find out when/if that part changed or if it was always that way.