Thanks to @Clement for sharing the broken version with us, we found the culprit:
This is related to one of the bug fix that has been done in 5.0 : Animation Group Recent Changes - #2 by sebavan Basically before the FPS in the loader was considered 1 meaning the animation worked on a frame basis instead of time
This is part of the breaking change list in 5.0 but I agree adding a meaning full workaround in the entry would be great. @carolhmj will update the whats new with the workaround:
BABYLON.SceneLoader.OnPluginActivatedObservable.add(function(plugin) {
if (plugin.name === "gltf") {
plugin.targetFps = 1;
}
});