I am using Blender to make animations through shape keys. Why does the face stretch appear after importing UE5.1, which is not consistent with what is expected in Blender?
Blender is an art program.
The engine is not.
Vertex deformation (positioning) has limits.
The limits are very high on artistic programs where high fidelity is a must, and really low on game engines where fidelity doesnt much matter.
In easier words, you are comparing apples to oranges by the way of apricots.
Simply: don’t expect the two to match - ever.
Look similar? Sure, to some extent.
The bigger the object, the more likely it will look and shift the same.
That’s likely also the same reason for your unreal UVs having a black bleed.
You can correct that one by increasing the UV fidelity on the mesh though.
There isnt a fix for the accuracy of how a morph target pushes verts around. It comes down to math.
Also, further from the origin of the world, worse the error will be.
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Hi there,
First and for all check of all scales are applied correctly for both shape keys.
if you want to keep the Shape Key function, you should check in Blender if:
A) there are any Constraints, any unapplied modifier or if you use bones too, you might check if correct weight paint was applied.
Or:
B) move between the Shape Keys in Blender and check if any unnatural movement occurs.
Maybe what you see in the picture is a result of importing the Shape kay at 50% interpolation.
Also check based on the way of Export, if the correct check marks are set.
Another way would be to take a different approach with your object.
for what I can see it looks like each pixel is a Face. Correct me here if I’m wrong. 
Since this looks like a 2D Model I would make a Render in both positions as a PNG with an Alpha Chanel and make a flip book out of it. means you use a plane and switch through different Areas of the Texture. subdivide the plane for some control regarding movement, if necessary.
Then a BP with a Function to switch between different states so you get a fluid animation.
Hope these pointers help.
Best regards
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