When I dismount and immediately start walking with W my avatar does not walk or run but glide over the floor with his legs not moving. Is that just a graphical glitch that only I can see or is that a bug?
I've had similar experiences both when mounting and dismounting. Can't remember the exact circumstances but I think it happens if I'm already moving when I mount/dismount using a keystroke, there's a slight pause before the animation triggers (but the character is immediately frozen); both the animation and frozen state presumably have a fixed duration i.e. neither is directly dependent on the other, so if the animation starts some time after the effect it will also end some time after the effect. I never bugged it but it probably should be reported, it's not just you who's affected.
I'm fairly certain that is a bug (I see it happening ALL the time). If you CAN however show how to replicate it happening and provide step by step instructions on how to replicate it, do post that as a bug report. I just have not yet figured out a way to 100% replicate it yet.