I'm on a Note8 and just upgraded from Oreo to Pie. This update lost my funtionality of being able to change the background in my messaging app. Looks like it's more an issue of the app on Android Version 9.
Hello.. I just change to Message+ from Verizon. I finally have an sms app that can be customized anyway you want and had more features.
My Verizon store had a lot of complaints about the messages app changed in the update so they been converting Samsung users to Message+ and they are like me happer now. Its also free with no ads