It wasn’t intended to explain: it was a comment on the use of the walk animation.
Considering the blending behaviour of the walk and run animations, it is unexpected (and probably unintended as well) for both of those animations to be the same, as they never usually are. Playing two of the same animation and blending them is probably producing this unexpected result.
I do find the behaviour a little curious though and I feel like looking into it on my own time. For the sake of clarification: you only changed the run animation and this got rid of any problems for you in regards to changing animations?
For future reference: playing two AnimationTracks which are of the same animation object asset ID at the same time results in buggy playback when their weights are blended together.
This is the most likely cause of breaking animations by modifying the default Animation script, typically due to the Walk and Run animation being set to the same asset ID. The Animation script will try to play and blend both of these animation tracks at the same time, but since they’re the same asset id, replication is buggy.
In the future you can test whether animations are actually being replicated properly by enabling in Studio: File > Settings > Network > Show Active Animation Asset. Within Studio, this will show all the animations playing for each humanoid/Animator object along with diagnostic data (animation weight etc). This helps you rule out potential problems because you can see how an animation is playing on a different client.
Sadly this is one of many grievances involved with using Roblox’s animation system for blending animations (we gain convenience, but we pay in weird bugs).
For PC it looks like the walk animation still plays at walkspeed of 16, but the weight is set to 0.00.
Note that this doesn’t just apply to the Animate script. I personally discovered this issue when I was trying to make some directional movement animations, but didn’t bother to make different assets for each direction. Never play multiple animations with the same ID at the same time, playback will be ruined on other clients.