Archmage 20/21 Threshold damage not changing

I had a lvl 19 archmage which i have corrupted to get the extra 1% of damage as lightning as you get an extra % when the gem gets to level 21 (using lightning mastery to get to 21), I would love to test if this works with a non corrupted level 20 spirit gem with a lightning mastery but i am not spending 6 div to test if it works.

the mana cost does seem to go down with the extra level

Hope that makes sense, I have attached an image which should help, maybe i am doing something wrong and i am stupid.

Link to image

https://imgur.com/a/gL9YBKd