Thank you for the quick response. I replied to your email. We managed to partially resolve the issue. It looks like the problem is in the pixi-spine plugin. The animations now work on Pixi.js 4 using pixi-spine 1.15.22 but they don't on Pixi.js 3 using the same version of pixi-spine despite both Pixi.js versions being supported according to the README. We have an error "Deform attachment not found: undefined". I will contact the authors of pixi-spine and figure out the cause with them. In the meantime, we will try another version of pixi-spine (pixi4-spine3.8 branch) which supports Spine 3.8 in both JSON and SKEL formats. I will contact you again if there is something on your side and will post in the forum as soon as we have a solution so that it can help others.
This is the issue in pixi-spine - https://github.com/pixijs/pixi-spine/issues/349
This is the broken sample animation - https://github.com/gatsinski/pixi-spine-sample