I don’t know if that might be related, but there are only 3 scenarios as far as I remember that would cause a character not being visible through the API:
- Character below level 10
- Character inactive for a while
- The new data policy: Data Protection Notice and FAQ
For the last scenario we still don’t know exactly what actions would trigger this. I think it would be a good idea for you to also open up a ticket just in case. It can take a while to get an official reply here.
I hope this info can help you solve this somehow. Good luck.