Dual wielding attack speed bug

The attack speed does not applied as average.
It is going to change speed randomly.
For example, if you equip two weapon, each has attack speed 1.0 and 2.0, it does not work as attack speed 1.5, but 1.0 and 2.0 was applied randomly.
I attached link that bugged attack video

https://youtu.be/W8GwPZMvFEA