Spark Cast Speed Mana Bug

Hi, i encountered this weird bug where when my cast speed is between 7 to 7.10, my mana drains ridiculously fast.

it doesn't happen below 7(even tested 6.99) or above 7.10.

Here's a video showcasing the bug where my mana drains normally at 7.14 CS but goes crazy at 7.09 CS

https://youtu.be/Or3zlDAMHqU