Last night, I configured Contact Posters for my in-laws. However, this afternoon, their contact information didn't automatically update, even after they participated in a group text (which I thought might trigger the update). To resolve this, I opened one of the contacts, selected the Contact Photo & Poster button, and then chose "Latest Photo (Update Automatically)," which immediately updated their information. I repeated this for the other contact, and it worked as well. It seems that these "automatic updates" aren't functioning as expected right now. Hopefully, Apple will address this issue soon.
Additionally, I noticed that Contact Posters may not be visible unless you have enabled Settings > Phone > Incoming Calls > Full Screen. However, most people seem to prefer the "Banner" setting these days because Full Screen used to take over your phone.