Cleanses that proc on gaining a condition, such as burning fire and transmute often (perhaps always) seem to only remove one stack if multiple stacks of a single condition are applied once.
I’ve noticed this pretty often but never had it recorded, but I caught it happening in wolfi’s video. At 3:21, you can see transmute converting 1 stack of confusion from a prybar into retaliation, while leaving the other 4 untouched.