Block subject uses block description instead of block title

The block subject is being set to the block description settings instead of the block title settings.

This might be hard to do an update hook for existing sites because site builders would have worked around this in various different ways.
If anything was to be automated, the thing that seems to make the most sense would be to set the block title to match the block description. The downside of that would be breaking things in the case where someone had overridden their site in such a way as to be already using the block title field. I expect that would be a very small (if any) amount of users.

Other than that, another way would be to leave block configuration alone and show a message to the user in an update hook to get them to manually check their blocks, along with a bold message in the release notes of the next release.