Hey Nate,
Thank you for your answer! Well it seems that this version of Spine is not supported by the Spine plugin for Phaser 3. In Phaser's documentation is written that the plugin is supporting version 3.8.95. And I did the animation with the latest stable version of Spine, which is 4.0.19. I tried to downgrade the export to 3.8, with presumption that it will convert all the animation and alpha curves to linear, but it didn't work. I am attaching the atlas and the error after downgraded version of the export.
My concern here is am I supposed to rework the animation in older version or there is a way to implement it with the current one?
Thank you!