They just cancel eachother out. Only reason they have both is for abilities which rely on buffs/debuffs. So Boba gets the extra damage, but net tenacity gain/loss is 0.
Also makes sense because they could be different durations. Tenacity down for two turns and up for one turn, for example. Makes more sense to show both for a turn and then up only for another turn, as opposed to showing nothing then having tenacity up appear out of nowhere.
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I was asking in the guild, but no one saw it before.
Thanks again :-)