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Sometimes missing key for mesh attachment
In the image you can see that key button is missing. it happens often. with any mesh attachment I still didnt figure out how to reproduce, but I know how to fix it 
just hide and show the attachment with the dot button next to it and key comes back.
screenshot from Animation view
I use latest stable version
I've been trying to reproduce that state on the example projects but so far with no results.
Considering that that key is supposed to key the deformation of the mesh, perhaps just moving a mesh vertex might make it reappear too.
Thank you for letting us know about it. Please let us know if you come across a way to reproduce it!