The bug
The width of the description of an advancement depends on the width of the first line, rather than the width of longest line in the description, or the width of the title of the advancement.
Example advancement
{
"criteria": {
"lock": {
"trigger": "minecraft:impossible"
},
"breed_two_cows": {
"conditions": {
"parent": {
"type": "minecraft:cow"
},
"partner": {
"type": "minecraft:cow"
}
},
"trigger": "minecraft:bred_animals"
}
},
"display": {
"announce_to_chat": false,
"description": "> lock\n> breed_two_cows",
"frame": "challenge",
"hidden": false,
"icon": {
"item": "minecraft:cow_spawn_egg"
},
"show_toast": false,
"title": "breed_two_cows"
}
}
Probably WAI, as normal advancements are supposed to have this function so they don't overflow, and the text box becomes too big. Once the top line is filled, the limit is set, so then the bottom text bits don't go any further than that.
It would be helpful, however, for this to be disabled if you use \n.
Can confirm, as well.