I'm still really new at blender and sim modding, so this may not help. I did have a problem editing an EA mattress, where it looked fine in S4S (and in blender) but in-game had weird mesh issues.
I think I was running into some kind of symmetry issue with the original mesh. Like, when I was merging vertices I was somehow also editing non-selected vertices. I ended up making sure that I never used "merge vertices" on anything from the original mesh, and only removed duplicates. That stopped the mesh error.
It's possible that if you started with an EA mesh, you're running into a similar problem?
The only other thing I can think to try is unwrapping in blender again, and reloading the mesh, in case there's some kind of leftover mapping information interfering?
It appears your weights are not right in the mattress.
Open your mesh in Blender and delete the mattress. Then append two meshes from the EA mesh you cloned (File > Append > Object > Shift-select S4Studio_Mesh_0 and S4Studio_Mesh_1). Delete the extra rig and the bone_bone_shape.
Go into front view (Numpad 1) then select your bed frame mesh and move it along the X-axis (G, then X). Try to center the legs with the EA bed's legs. You may want to resize the bed frame a little too so that the mattress doesn't clip. Select the bed frame, then press S to scale, then X to constrain it to the X-axis. You may also need to go into edit mode and move the sides of the bed inward along the X-axis, as well. What you do with the bed frame to get it to stop clipping through the mattress is up to you, to be honest. You can resize the mattress to make it wider but I think it will affect how the animation will look. I haven't tested this, however. My advice is to not touch the mattress and adjust your bed frame, instead.
Once you have your bed frame adjusted so it's not clipping through the mattress, delete the EA bed frame. The mattress should already have the correct cut number. You can go ahead and save it and make your lower LODs, as usual.
Edit: Forgot to mention. Don't move the mattress once you append it into Blender. It'll automatically be positioned where it should be and if you move the mattress it'll affect the animation. Basically, append the mattress and don't touch it. Append the EA bed frame, too and use it as a guide for where to position your bed frame.
Another thing I forgot to mention. You'll need to adjust the size and/or position of the flat plane meshes so that they match up with the changes you'll make to your bed frame.
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