Why does this animation's weight slowly increase over time as I run?

I have a custom walk animation and put it in the default Animate script, and it works well, but if you were to walk for several seconds, then the animation would slowly get slower over time and look weird.

https://gyazo.com/ff0075d968def59bdb36f2310c43514e

Can someone explain me how to fix this issue?

Another example: https://gyazo.com/58db40efb14f9ced1abffbab2f8195bc