Blender to Unity: Animation is wrong applied getting animations to Unity is usually not a problem at all, but today I tried something new.

I animated this 4-parted horizontal sliding door by creating ONE armature with 4 bones for each part of the door. Then I parented each door-part-object to the correct bone. Everything is working fine.  Once I export this to Unity, I get a couple of clips, which is fine, but none of them contain the animation.



Instead, each clip moves all 4 door-parts at once. enter image description here Does anyone have an idea how to fix this problem? Thank you in advance!
I made a workaround:

Instead of weighting (CTRL + P > Bone) for each door-part-object, I joined them to a single one and applied ( > With Automatic Weights). Than removed the vertices from the vertex groups and asigned them manuelly. 
Now the animation worked as expected in Unity