I don’t think there is a supported way to prevent this.
If you consider this an issue, please open one at https://github.com/getkirby/kirby/issues and Bastian can see whether he agrees, since he is the most familiar one with the list field.
I don’t think this a ‘neutral’ implementation of a list (if Bastian agrees or not ;-). Those p tags simply don’t belong in a short list of some kind.
I’m not a github user …