Several changes were made to the MetaHuman assets that are exported to Unreal Engine and Maya, beginning in Unreal Engine 5.2. These changes break backwards-compatibility with previous versions of MetaHumans in order to accommodate the live capture capability of MetaHuman Animator. This means that your custom Metahumans will not be visible in Unreal Engine 5.2. You can change the version of your Unreal Engine from the top right corner:
If you launch version 5.1, you'll be able to access the custom Metahumans made to be exported to Unreal Engine 5.1 (only).
If you want to upgrade the MetaHumans inside your Unreal Engine project without upgrading the project itself to a newer engine version, this is currently not possible. The newer versions of MetaHumans only work with the matching version of Unreal Engine. For example, the version of MetaHuman for 5.2 only works with Unreal Engine 5.2 or later.
To find the steps to upgrade your Metahumans to be compatible with Unreal Engine 5.2, you can find an in-depth guide right here.