Why don't Animation Timeline show any object on "Add Property"?

Sorry for my poor English.

I would like to know what conditions are and are not displayed as candidates in “Add Property” in the Animation Window.

I am currently creating my own Animation.
I selected a bone object from “Add Property” in the Animation Window and created animations, but for some reason the bone object is no longer showing up in the Add Property list.
When I loaded the FBX into the scene again, it was displayed there, but when I opened Unity the next day, it was not displayed anymore. If I put the bone object inside a mesh object that is displayed without problems, it appears, so I don’t know what is causing it to disappear.

Can you provide some screenshots or videos?

“Chr_Mae(1)” is my character to animate.

When trying to organize a character’s body parts in an empty object (GameObject2), some objects do not appear in Add Property (GameObject in this case) Assume that the GameObject contains bones! The GameObject is not a bone.
This symptom will only show up when you restart Unity after changing the parent-child relationship.

Do not mess with the parent-child relationship, and take the body object out of the empty object once, and all the objects will be visible again.

Game development is not impossible, but the symptoms and cause are beyond my comprehension, so I need your wisdom.