Looks like Comet ATLAS was a dud.  It broke up into pieces after it had brightened much higher than predicted last month.  The brightening and breakup so soon afterwards indicates a large release of material from inside the object as it cracked open, so to speak.

By the time I took this pic, there were at least 3 to 5 major pieces in a line and the out-gassing of volatiles available was over with.   Just rock and dust was all that was left.