I figured this out in the end. Googling didn't give me any results regarding a custom Azure index but it did give me some ideas on what might be up:
Experience Analytics are not showing Online Interactions
These links mentioned issues with configuration.
It turned out to be a problem with how one of my custom fields was configured in my Azure indexConfiguration file. e.g
<field fieldName="customfield" cloudFieldName="customfield" searchable="YES" retrievable="YES" facetable="YES" filterable="YES" sortable="YES" boost="4f" type="System.String" settingType="Sitecore.ContentSearch.Azure.CloudSearchFieldConfiguration, Sitecore.ContentSearch.Azure" cloudAnalyzer="language" />
If you have this issue things to check in your config are:
- Check your field types are of the correct types - and that you are mapping them to the correct Azure Field types.
- Check your cloudFieldName is in the right format for each field - It can only The first character must be a letter. it can only contain letters, numbers, and underscores.
- Check your settingType is correct
More info here: https://doc.sitecore.com/developers/82/sitecore-experience-platform/en/configure-azure-search.html