Has not happened a single time to myself or anyone in my alliance.. The second Ultron is at 0 health, the bombs poof BEFORE any tile cascades or timers.
Not sure why you seem to be the exception, but i’ve killed him enough with board shakers etc to see that poof before the board shakes when the ability kills him.
Were the bombs bubbled? Maybe there’s some conflict since a bubbled special tile is pretty much in limbo and doesn’t operate normally. Maybe when Ultron died and the bubbles were burst, whatever fix wasn’t applied to disable them?