When your project uses Auto-Export, but also in order to correctly migrate the HST channels, you may need to upgrade your project's configuration of the Auto-Export feature.
For CMS 12, the Auto-Export feature has been extended with functionality to automatically inject repository-data categorization meta data, especially the .meta:residual-child-node-category property to indicate that new child nodes created for a specific node should be treated (and exported) as category content. Where to inject this meta-data can be configured using the autoexport:injectresidualchildnodecategory property.
Also, CMS 12's Auto-Export feature is capable of overriding the system's (residual) child node category for the purpose of exporting. This mechanism is controlled through the autoexport:overrideresidualchildnodecategory property and also documented for the Auto-Export feature.
The Hippo product by default contributes our best practice configuration, setting several injection and override patterns especially for HST configuration. If your project used non-default settings to control the injection and overrides of .meta:residual-child-node-category while migrating the bootstrap resources, these settings should be configured for AutoExport, too.