Thanks for the response.
I don't even know where to start.
In short, I haven't been able to solve the problem, even though I've considered different options, from changing the stairs to altering the character's height. In general, none of that worked.
I disable player control and switch it to the game AI. Simply put, at that moment, the player becomes an NPC, who is then controlled by the quest scene in which they participate.
As I mentioned, I'm creating a quest, so yes, it's a mod and not a vanilla game.
And no, I don't change the player's speed with scripts. I just activate the AI package in the scene that tells the player to move to a specific point with a "preferred speed" of walking.
The problem is strange because it only occurs when the AI controls the player but doesn't appear with NPCs.
If I manually controlled the character as a player and walked slowly down the stairs, there would be no problem.
The only thing I haven't tested yet is creating another scene with a different descent in a different location to check whether this problem is consistent or just local.