If you currently save a preset with the same name as another one and try to modify it, both will be modified. I understand that it uses the name as a primary key and that is where the problem lies.
This is highly problematic if you usually save a new preset from another one, as it renders a slot unusable. This is especially serious if you have both assigned to a footswitch.
The only way to fix this is to do a factory reset.
I have the exact same problem. please fix this in an upcoming update.
Renaming the slot afterwards does not fix it. After a restart of the NC the presets are named the same again