Unfortunately yet another issue popping up. Recently my (and my partners) bullseye tool refused to work. The toggle lights up and turns off but still the zoom tool remains.
I think you're referring to the targeted adjustment tools, and that's a known bug in CC 2015.10.1: Lightroom CC 2015.10.1: Develop module: Target Adjustment Tools not working | Photoshop Family Customer Community
You can roll back to 2015.10: Lightroom roll back to prior update version
Some have also reported that if you collapse all the Develop panels except the one containing the tool you're adjusting, the problem doesn't occur.
Please add your me-too vote and detailed opinion to the bug report linked above, and you'll be notified when the bug's status changes.
Well, looks like I'm not the only one....so many bugs lately. :/ Hopefully this gets resolves soon.