Hi-
I’m not sure if there is a particular pattern to this. Based on some examples I’ve found, it may be characters who were created after a certain date (?), but this is only a hunch based on a very small number of characters who I am familiar with and may not be accurate for all examples.
Some examples:
wow/character/moon guard/abraaegis?fields=achievements&locale=en_US&access_token=
/wow/character/moon guard/aelk?fields=achievements&locale=en_US&access_token=
I have a list of a few thousand of these if it would help narrow down the issue.
Thanks for your work on maintaining the API!