As per: https://forum-en.gw2archive.eu/forum/info/updates/Game-Update-Notes-January-26-2016/first#post5946360
Fixed an issue that allowed players to skip the Molten Effigy boss in the Molten Boss fractal.
I have no issue with this change, however… If this boss cannot be skipped, how about removing “Bonus Event” announcement that comes up. Bonus Event makes it sound optional. Or changing it to “Mandatory Event” if it has to say anything.