Why does viber keep reverting to default chat background?

I have a Samsung Stratosphere II. Android version is 4.1.2, which is the most up to date version of the software my phone model can get. I am running viber version 5.4.0. I recently updated viber and noticed the other night that my customized chat background is now constantly reverting to viber's default chat background. Even if I do not close the program (clearing it from task manager or exiting via within viber), after a certain amount of time (running in the background) it reverts to the default background.

I only noticed this problem last night and it hasn't been a problem previously. How can I get my custom chat backgrounds to stay?