After extensive research I was able to find the solution to make the .json files made with spine 3.8 compatible with the game!
You must be registered to see the links
is a tool that allows you to rollback them to 3.7
You have to use this command in a console to use it:
java -** skeletonViewer.jar com.esotericsoftware.spine.JsonRollback input.json 3.7 output.json
Here is a video explaining how to use it and the website of the tool
You must be registered to see the links
You must be registered to see the links