I’m not sure why this is, but if it bothers you just delete the keyframe in Unity after importing your fbx. Unity won’t create an Animation Clip at all unless one of the standard attributes (translation/rotation/scale) is keyed.

Your Maya animation will need at least one keyframe on a standard attribute for Unity to read your Custom Attributes.

Just a single keyframe will suffice, just as long as there is something there to pass into an Animation Clip.

Unity will only import Custom Attributes if they have animation data assigned to them.

Two last important things you’ll need to remember about preparing your asset for export:

If you pick any other type of data Unity won’t import it. In Maya you have a range of data types to choose from when making your Custom Attribute, but in Unity you have only one, the float curve. When exporting your asset the fbx exporter will use the attribute’s long name, so make sure you name it something you’ll recognize on the other side. You’ll also be given the opportunity to create a custom nice name, which is intended to be a more human-readable (and animator friendly) version for the Channel Box. This is a string which acts as an identifier for the attribute. When you go to add your custom attribute, you’ll be asked to provide it with a long name. The main mesh of your object or the root joint of a character are logical spots. My suggestion would be to add them somewhere easy to find, especially if you’re planning on adding more than a few. You could apply it to a joint, directly onto your mesh, or even to a null object like a Group, just as long as it exports to fbx. Unlike preparing your asset for the Unreal Engine (in which skeleton hierarchy is more important) it really doesn’t matter which object to which your Custom Attribute is attached. To create a Custom Attribute in Maya all you need to do is select the object to which you want to assign the attribute, and select Add Attribute from the Modify menu. This procedure works in any version of Maya that can export to fbx, including both the educational version and MayaLT. Although this process can get complicated, for our purposes there are only a few steps we need to take. The fbx format is incredibly versatile, and the pipeline for bringing an fbx file into Unity from Maya is very well documented. This procedure will transform any Custom Attribute attached to an exported object into a float Property that can be read by that object’s Animator component. Importing Custom Properties from Maya into Unity using the fbx format is a simple process as long as the animation data meets a few basic requirements. Custom Properties have been a core feature of Maya for years, and they have become ubiquitous in the fields of rigging and animation for their power to transfer animation data between objects in your scene.

Maya fbx export options for unity