Self contained reproduceable bug:

make init

https://localhost/api/entry_points/1

api_platform
    normalize_child_class_attribute_groups: true/false

{
    "@context": "/api/contexts/EntryPoint",
    "@id": "/api/entry_points/1",
    "@type": "EntryPoint",
    "id": 1,
    "name": "entryClassName 1",
    "joinedTableEntities": [
        "/api/joined_child1s/1",
        "/api/joined_child2s/2"
    ]
}

• Serialization OK, joinedTableEntities should be iris

https://localhost/api/entry_points/1?groups[]=normalize_with_attribute_in_parent

api_platform
    normalize_child_class_attribute_groups: true/false

{
    "@context": "/api/contexts/EntryPoint",
    "@id": "/api/entry_points/1",
    "@type": "EntryPoint",
    "id": 1,
    "name": "entryClassName 1",
    "joinedTableEntities": [
        {
            "@id": "/api/joined_child1s/1",
            "@type": "JoinedChild1",
            "name": "child type 1"
        },
        {
            "@id": "/api/joined_child2s/2",
            "@type": "JoinedChild2",
            "name": "child type 2"
        }
    ]
}

• Normalization of joined child OK, attribute is found in parent class:

https://localhost/api/entry_points/1?groups[]=normalize_with_attribute_in_child

api_platform
    normalize_child_class_attribute_groups: false

{
    "@context": "/api/contexts/EntryPoint",
    "@id": "/api/entry_points/1",
    "@type": "EntryPoint",
    "id": 1,
    "name": "entryClassName 1",
    "joinedTableEntities": [
        "/api/joined_child1s/1",
        "/api/joined_child2s/2"
    ]
}

Normalization of joined child KO, normalize_with_attribute_in_child is indeed a requested group found in child class, but not serialized

https://localhost/api/entry_points/1?groups[]=normalize_with_attribute_in_child

api_platform
    normalize_child_class_attribute_groups: true

{
    "@context": "/api/contexts/EntryPoint",
    "@id": "/api/entry_points/1",
    "@type": "EntryPoint",
    "id": 1,
    "name": "entryClassName 1",
    "joinedTableEntities": [
        {
            "@id": "/api/joined_child1s/1",
            "@type": "JoinedChild1",
            "customClassTwoField": "custom string 2"
        },
        {
            "@id": "/api/joined_child2s/2",
            "@type": "JoinedChild2",
            "customClassOneField": "custom string 1"
        }
    ]
}

Normalization of joined child OK, normalize_with_attribute_in_child is now found when searching in JoinedChild1 and JoinedChild2 classes.