Currently, there is an issue where the optimization slider may go back to "Balanced" when you optimize a game for Performance or Quality, and then do another action like switching to another game in the library.
Thankfully, the actual optimization should not change. (This is an example of Minecraft which was set to Performance before moving back to Balanced):
However, if you would like to optimize for balance again, a quick temporary fix is to enable the "Ignore Optimizer" checkbox and then to disable it.
Additionally, another symptom of it going back to Balanced may just be that what you have optimized it to has no graphical difference between it and Balanced. For example, if you were to Optimize Hearthstone to Quality, it may fall back to Balanced if it did not need to change anything.