NiaSwift

This appears to be a duplicate of MC-301271, which has been fixed in a recent snapshot.

Oh, sorry, I didn’t know it was already reported and fixed. I did look for duplicates before making this report, but I guess I didn’t look well enough.

Also, I wasted so much time on making this report, and it was all for nothing 😭

I just discovered this bug on my own, only to find out a few other people have already discovered it lol.
Also, the advancement "The Healing Power of Friendship!" has this issue even when using English (US). In the toast and in chat, it says it with the exclamation mark at the end, but in the advancement menu it is without the exclamation mark.

I also discovered that it's not the amount of characters that limit it, instead it is the length in pixels.
When using GUI Scale: 1, the text "The Healing Power of Friendship" is 161px long (note that it is without the exclamation mark). The exclamation mark adds 2px, which would make it 163px long, which is apparently too long for the Advancement Menu.

However, if you use a resource pack to change the name of the advancement, you can actually get it to show text up to 162px long. For example, the letter 'p' is 6px long, but the letters 'k' and 'f' are each 5px long. So, if we replace the 'p' in "Friendship!" with either a 'k' or an 'f', we will be reducing the length of the advancement's name by just 1px, which is just enough to show the whole text in the Advancements Menu, including the exclamation mark.

To test this yourself, you can simply create a resource pack, and in RP/assets/minecraft/lang, create a text file named en_us.json, open it and copy paste in the following text:

{
  "advancements.husbandry.kill_axolotl_target.title": "The Healing Power of Friendshik!"
}

Then, reload the resource pack by using F3+T and give yourself the advancement with the following command:

/advancement grant @s only minecraft:husbandry/kill_axolotl_target

Finally, open the Advancement Menu by pressing L on your keyboard, and go in the Husbandry tab where the advancement is. It is nearly impossible to tell by just looking, but you can take a screenshot and use an image editing software to zoom in more, and you'll see that compared to how long it is without the resource pack, it is 1px longer (counting the shadow of  the text too, of course).

Just found this bug by accident, came here to report but I guess someone else has already found it. Also, this still happens in 1.21