Thank you for your report.
This is an inherently very complex problem to solve, as this would require a very deep pathfinding algorithm to check whether there are any other roads to connect to an existing city. The current system for connecting industries with roads relies on existing code from Standard, so this addition was intended to be lightweight.
If anyone can come up with a working solution to this, that would be splendid, but I expect that this would be a major project.