Well, the variables Learns_{skill} have the ActorId of the character who learned that Skill.
The Common Event 505 Spice_SkillSET if the variables forge_{skill} have 1, 2, or 3. 1 for First Attack, 2 for Double Attack, 3 for Greater Attack, then the Common Event Gives the Actor stored in the Learns_{skill} the upgraded ability.
An possible cause of the problem is that the game have a true starting character, that is Levi. Once you press 'New Game' in the menu, inside the codes of the game you already have Levi as Party's Leader, and since the LeaderID of your plugin is a constant, the Plugins always consider Levi as the Leader.
I'm having an error trying to send prints, so I will send the prints though my Google Drive.
https://drive.google.com/drive/folders/1d-CFeWBoH1sInYSu-REvpcPrxzdDC2ta?usp=sha...
Sorry for the music in the video, i like dark souls music.
Edit 2: I tested and now i have certain, the proplem is that the game have a initial party, and levi is in that initial party, so he end being considered the Party Leader. Watch the video in 'cause'