krinn opened the ticket and wrote:

Change to openttd (i'm using still 1.3.0) to handle difficulty setting now bypass AICONFIG_INGAME (well when not present i mean) and author restriction to alter a setting in game.
Use any AI with that setting :

AddSetting({name = "fairlevel",

description = "Alter how the AI act with others",
min_value = 0,
max_value = 2,
easy_value = 0,
medium_value = 1,
hard_value = 2,
custom_value = 2,
flags = 0
});

Now change difficulty in openttd competitor setting (the AI profile for difficulty), even kept gray, the fairlevel now will display the state of the new difficulty you have pick.
So it disable the flags that is set to not allow to change that setting.

Reported version: 1.3.1
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/5644