The issue has been introduced by #122105
The image save operator does exactly the thing which was not supported by
the change and was not caught during development or view: it acquires and
releases image buffers while a render result of the Image data-block is
held acquired.
The solution is to implement a simple user-counter for the render result.
Currently it is only used by the image's render result acquire/release API,
as it is the most important case for now. The render pipeline uses its own
way of protecting the result, for which the user counter is not needed.
Pull Request: https://projects.blender.org/blender/blender/pulls/122231
62ebc5f351