Make sure you understand how the Victory Bonus works in general: How does the Victory Bonus work?
Experts might have observed that there are situations, most prominent during season resets, where the multiplier may charge inconsistently visually. The system behind the multiplier is pretty complex to ensure it works with a wide range of Skill Rank and heroes.
The visual charge of the multiplier is relative to the hero with the highest Skill Rank. It appears that if you rank up a hero from 10 to 11, the multiplier charges slower if for example you have another hero with a Skill Rank of 40.
That is because the the speed at which the multiplier is consumed is aligned to the Skill Ranks of your heroes.
Please note that the math behind the system makes sure that you can Skill Rank Up in any order you want, play with any hero you want. You always get the same overall sum of Victory Points from the multiplier.