In the messages.yml config, the placeholder %embedavatarurl% is listed to return the image url to the users avatar, but for the advancement configuration it returns an image of the item/block related to the advancement.
I would still like to see this functionality kept as it is very cool but maybe as a different placeholder specifically for the advancement config.
I was trying to configure the advancements embeds so that the users avatar would display as the author image in the embed and move the advancement image to the thumbnail image of the embed when I discovered this.

Thanks :)

Nevermind, I found out that it was another plugin that changed this called InteractiveChat DiscordSRV Addon.
Sorry for the inconvenience.