The spell does a check every tick (6 seconds) to see if it should break, based on some equation that factors in charisma, level difference, and the mobs MR. I am 80% sure charisma is "taken out" of the equation completely at 200.
Early charm breaks on an otherwise 'charm friendly' mob don't really give any indication of anything in particular going wrong. It has the same chance to break the first tick as it does the 150th tick. And because time is linear, it just seems like bad luck.
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