You are passing the result of a make_uuid call instead of a callable object to default.
Your code equals the code below.
default_uuid = make_uuid('AC')
account_uuid = models.CharField(max_length=34, default=default_uuid, db_index=True, unique=True)
The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.
You can use partial or lambda to generate a new uuid function.
from functools import partial
account_uuid = models.CharField(
max_length=34,
default=partial(make_uuid,'AC')
)