The related record of a feature layer shown in the web map in ArcGIS Online does not appear in the pop-up of an ArcGIS Online Story Map.
This is expected behavior. An ArcGIS Online Story Map does not display a relationship class of a feature layer in a pop-up. The pop-up display in story maps is simplified to reduce excessive information, buttons, and links from appearing in a pop-up; in this case, related records are not displayed as the records are primarily used for analysis.
As a workaround, create a web app from the web map containing the related record with Web AppBuilder for ArcGIS. Embed the web app to the story map by the following steps:
Note: The interface to embed the web app may vary depending on which Story Map template is used. This workflow uses the Story Map Journal and Story Map Series templates. For more information on the interface to embed a web app in a Story Map Series, Story Map Journal, and Story Map Cascade, refer to ArcGIS Blog: Embedding a Story Map within a Story Map.
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