When an item contains more than one link field, the Layout Service might render link fields as lists of IDs. Maximum depth is controlled by the serializationMaxDepth setting defined in App_Config\Sitecore\LayoutService\Sitecore.LayoutService.config by default. When this value is reached for at least one link, all other links are rendered as IDs without any additional data, like field values, even if the maximum depth has not been reached for these links.
The issue affects JSS 14.0.0 and 15.0.0 versions.
To resolve the issue, consider one of the following options:
Upgrade Sitecore XP instance to JSS 15.0.1 or later.
Download and install the following hotfix:
Be aware that the hotfix was built for a specific JSS version, and must not be installed on other JSS versions or in combination with other hotfixes. In case any other hotfixes have already been installed on a certain Sitecore XP instance, send a request for a compatibility check to Sitecore Support.
Note that the ZIP file contents need to be extracted to locate installation instructions and related files inside. The hotfixes must be installed on a CM instance and then synced with other instances using standard development practices.