i created a perk that gives me more carry weight, but the Message keeps popping up telling me that i'm overencumbered before i actually reach my new carry weight limit. it pops up each time i pick up any item while i'm over 300 weight, but under my actual limit of 500

i've check around creation kit but couldn't find anything that would change when this message is displayed

Yep, it's an issue unfortunately. No solution I know of, except using an Ability instead.