Game Mechanics: Characters not gaining experience

[Psiko]

Hello, I logged into my account this week for the first time in roughly 5+ years. I noticed my characters are not gaining experience when I kill something, even though it says I gained experience. I tested this killing multiple mobs using a level 5 alt and a level 15 alt. I'm not sure if this has anything to do with it, but I installed this on a computer using Linux Mint.

Thank you for your help,
Psiko