Warning "ItemA is being serialized by [SerializeReference], but is missing the [Serializable] attribute..." appears when SerializableReference is correctly used in a script

Unity: Serialization

6.5.X

6.4.X

6.3.X

6000.3.0a5 onwards

6.2.X

6.0.X

This warning does not exist in 6.0

How to reproduce:
1. Open the attached project "IN-125546"
2. Observe the Console

Actual result: Warning appears in the console
Expected result: No warnings appear in the console

Reproducible with: 6000.0.63f1, 6000.2.14f1, 6000.3.0a5, 6000.3.0f1 (d1870ce95baf), 6000.4.0a5, 6000.5.0a2
Not reproducible with: 6000.3.0a4

Reproducible on: Windows 11 Enterprise
Not reproducible on: No other environments tested

Note:
In the included project, see the “CustomAsset” script which demonstrates valid use of a SerializableReference.