Hi,
Both the heirloom and toy collection API’s doesn’t work for certain characters on the same account.
Example working (old character i only login to sometimes):
https://eu.api.blizzard.com/profile/wow/character/kazzak/thedrunken/collections/heirlooms?namespace=profile-eu&locale=en_EU
Example returning 404, is my main character I login every day:
https://eu.api.blizzard.com/profile/wow/character/silvermoon/tovven/collections/heirlooms?namespace=profile-eu&locale=en_EU
Same goes for the toys and pets API
But the mount collections API work for both characters