Tonight I hosted a party for the first time since the September 10th/11th version 16.0.0 update for Fashion Week.
I don't know what changes were made to the parties, if any, but my guests and I saw a noticeable improvement in sim rotation, at least within the first 10 mins. (Since about 25 friends attend my parties right when they start to maximize visibility for getting stickers). It was amazing seeing more sims than I usually do without having to enter/exit CAS and close/open the app. After ten mins, all the sims ghosted always; however, I was able to dispense my stickers quickly to everyone at the start.
I won’t jump the gun and suggest this bug is fixed (yet), but it looks like maybe some good tweaks were made.
Unrelated to the bug, the text/font/size in the party chat typing box definitely changed, and that was a big improvement!