To address your request, you would need to manually add the Dispose process for buffers in the OnDestroy method of SkeletonPartsRenderer in your 4.0 version of the Spine Unity runtime. Since the 4.2 version includes this feature, you can refer to the implementation in the 4.2 version and adapt it to your 4.0 runtime. This involves adding the necessary code to release any resources or buffers that need to be disposed of when the SkeletonPartsRenderer is destroyed. Unfortunately, I cannot directly modify the runtime for you, but you can follow the example from the 4.2 version to implement this change in your project.

@masuda_takaaki Sorry to say that 4.0 runtimes will not receive any updates any more as they are several years old and also shall remain stable as-is. As Spinebot suggested, you should be able to integrate the respective commit in your own project (repository) rather easily.