The Sparkmaster buff granted by Sparking Swings sometimes wears off much faster than the listed duration of 3 turns.

It seems to wear off after used a set number of times, regardless of how many turns have passed, but there is no mention of this functionality anywhere in the buff or skill tooltips.

So in the StatusData for SparkingSwings (Sparkstriker), it actually only has 4 charges - This means it's gone after 4 hits. I agree, they should mention that in the description.

There's a Sparkmaster status that's similar, but has -1 charges, meaning it doesn't run out after a certain number of hits like the weaker version does.

Please report this as a bug in the technical forum, because it's clearly not how it's intended to be. Either it works the wrong way or the tooltip is wrong. Either way, it needs to be fixed.