Spine processes the images path and makes it relative if it is within 3 parent folders. This limit of 3 parent folders was chosen somewhat arbitrarily, but we do need a limit so you can use an absolute path. Without a limit you'd end up with something like
../../../../../../and/then/the/path if you chose a folder very far away from your project folder.
Spine does this path mangling automatically, without a way to preserve the path as entered by the user. Usually this isn't a problem, you are the first to mention needing a relative path with more than 3 parent folders. Unfortunately we don't have a workaround at this time, so organizing your folders to avoid needing 3 parent folders would be a good solution.
Another option could be to use symbolic links. These are common an Linux and Mac, but they can also work on Windows. For example, each of your Spine project folders could have a symlink called
images that points to some other path (relative or absolute). It appears to Spine like a normal folder.