External ID not getting saved in topic created by API

I’m importing topics with a script and trying to set the external_id for each topic but it’s not working. I can see that the external_id field is included in the POST to create the topic. The topic is created successfully but when I look at the new topic in the database the external_id field is NULL.

Here are the request headers & payload.

Any idea why the external ID isn’t being saved with the new topic?