It is also in the API section when it should have been posted to the Minecraft Bug tracking section.
This is not an API feature request. This JIRA is for API feature requests only.
For bug reports please use the MineCraft Bug JIRA
For game feature requests/changes please use the MineCraft Forums: Suggestions
Also, you should follow the standard bug report format when you submit this to the correct place.
Since the top-slab is not a whole block, I would think it shouldn't transmit redstone signal at all. So, what I would expect to happen would be that the trap door doesn't move at all.
I think the bug here is that an open one is closing.