PIS is when the characters act dumb because it benefits the plot (ex, not using one of their abilities despite having a clear chance to use it when it would make victory easy). Powerscaling issues are outliers. It's a bit of a pet peeve of mine when people confuse the two.
Anyway, in my opinion, an outlier always occurs when the powerscaling makes no sense. I don't care how often a character is shown to be able to fight beings who should be vastly superior to them. No matter how many times they defeat such beings, if powerscaling says they shouldn't be able to do that, I consider it an outlier. I am a very "mathematician" kind of guy when it comes to powerscaling. I always go by the numbers regardless of what's actually shown. For example, I consider Dragon Ball Super to be filled with outliers simply because the powerscaling makes no sense. So, how often Twilight can contend with certain beings is kinda irrelevant. It only matters if it makes sense. If she appears to have gotten way stronger, the power increase needs to be explained and made believable for me to not consider it an outlier. For example, deflecting Sombra's blasts before she was able to use the power of friendship to destroy him is something I consider an outlier. I don't believe you can convince me otherwise since the characters kept saying she stood no chance in a direct fight.