Finally found the problem.
There is actually a double condition to satisfy.
First one is that the FBX and the alembic files have to begin at frame 0.
By default when you ask the Maya Pose Generator to generate and export your fbx+abc for let say 1000 poses, it set up your timeline from 0 to 1000.
And when you put those 2 files in Unreal, it says in the MLDeformer asset that those file does not have the same length.
Weird thing but it seems that it always exports my abc on 1001 frame and my fbx on 1000 frame and MLDeformer yell at me that length of both files are not the same…
So first things first you ALWAYS have to export your fbx and abc by hand to ensure that you have the good animation length and starting frame 0.
Secondly, I found out that Unreal is unable to interpret Step Mode animation if the animation is not at 30 fps. And everything for my project was setted up at 25 (in Maya & Unreal) and ML Deformer use the step mode to train.
So you must generate your Maya poses and export everything at 30 fps and everything should work. (plus, you should force “custom sample rate” at 30 when importing the fbx in Unreal).
EDIT: And lastly, I also found that the option “resample all” have to be unchecked when you export the animation in FBX otherwise you will have some mismatch between your animation and your baked animation in your fbx.
2nd EDIT: When you export the alembic from Maya, both “UV Write” and “Write UV Sets” option have to be unchecked otherwise strange things happend in ML Deformer where your deltas changes every X frames. Dunno why exactly. But I finally have a ML Deformer working. it was a huge pain… took me one month.
Hope this help.