I'm in the middle of attempting to upgrade to UE4.20.3/3.7-cpp-beta and I ran into a problem that appears to be related to mesh deformation (see screenshot), but I can't be sure. Basically, there is an intermittent distortion that happens while attempting to render an animation in UE that doesn't happen in the Spine client. I slowly whittled my main character down to just single animation with two parts and created the attached repro using the newest Spine runtime/examples in git (using branch 3.7-cpp-beta).
In the repro.zip you'll find the Spine project file, export files, and all the UE4 files. Please let me know if there is anything else I can provide to help nail this down. It's a show-stopping issue for us at this point as it effects all of our existing characters in one way or another.
Link to Repro:
http://www.aeriedigital.com/temp/Repro.zip
Keep in mind you may need to run the UE project multiple times before seeing this issue. It is somewhat intermittent , but within 20 run you should definitely see it and usually within the first few times.
Screenshot:
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