By default, bp_create_excerpt (found in /bp-core/bp-core-template.php) sets a default length of 225. The Achievements plugin calls it in achievements-templatetags.php, causing descriptions longer than 225 characters to be truncated.
As a temp fix, I copied bp_create_excerpt over to bp-custom.php, increased the $length, renamed it my_bp_create_excerpt, and then renamed it within the Achievements plugin as well. (I’m sure there’s a better way to change the $length, but this works for now)