After you increase the partition size, you also have to increase the size of the file system on that partition. Don't worry, this is pretty quick in comparison to the other stuff I made you do.

Enter resize.f2fs /dev/sdXY where /dev/sdXY is the Partition file. In my case it was /dev/sdb15

If everything goes correctly, the command finishes pretty quickly.

If you get Mount unclean image to replay log first that means the partition wasn't unmounted correctly. Just mount and immediately unmount it in your OS and try again.

If you get Error: Device size is not sufficient for F2FS volume, more segment needed you might be using an old version of f2fs-tools. There's a bug that has been fixed with version 1.15. That's what happened on my machine, and is also the reason why the console looks different in this screenshot.