The button does not work on currently researched technology.
But it would be useful to reset the queue and keep this current technology in research.
For comparison, the Set Research button is enabled for other queued techs and works as expected (resets the queue and sets the tech for research).

Just set the research on another Tech, the jump back to the current tech. No research is lost since each tech actually has a pool that can be filled whenever you feel like it (good work devs, hate it in other 4x games when you loose research just because you need to change research). You know also that you can build a research que by using the shift button and press directly on the tech in the tree (not use the set button).

A feature I would like to see in the tech tree is to "unmark" the current que. The only way to do this today is by using the trick above, but I would like to see (still holding down "shift") being able to unmark things in the que.

You can accomplish the desired action by right-clicking on the currently researched tech, but I can see why it might make sense for the button to do the same thing.