[GamePlay] Janna's Movement Speed drops because of the "Boot of Mobility"?

When Janna activates the "Boot of Mobility" with(/without, in this case I have learnt W) her W passive skill which will increase her movement speed as well, the bonus speed(+105) from the "Boot of Mobility" will not stack onto the accumulated speed but instead will only stack on her base speed movement and ignore any skills or runes or masteries which will give movement bonus to champion.

501 is the movement speed before attack the monster while attacking the monster, Boot of Mobility is "activated" and drop my movement significantly to 426. Leaving the battle stage and wait for 2 second to cooldown the item and the movement speed goes back to normal(501).

Hope this can be fixed asap because Janna needs to run away when things go wrong. !!

ps. for some reason i cant upload pictures to show. tell me if u need pictures