Yeah, it looks like there's no way to fix this... I'm looking for an alternative way to save data.
For those who find this post after searching this problem on google, I'll be editing this answer to include any interesting stuff I find.
I'm currently trying to follow this -> https://unity3d.com/de/learn/tutorials/topics/scripting/persistence-saving-and-l... It's kinda old but I think it still works in newer unity versions. This doesn't work because it involves creating a file and, for security reasons, WebGL is not able to write files. (It's still a good way to save files on other platforms though).
I'm still searching for PlayerPrefs alternatives.