There was a fix for this on kRO as you can see here. iRO will need to request this bug fix along with others from kRO.
That fix cites the Rogue card set, not the thief card set. It's not clear whether the bug fixed there is the analog of what happens with the thief card set, or whether it merely fixes an issue wherein the snatch autocast wasn't disabled ever.
I think the issue is that the set is implemented as:
Set effect: 0% chance of autocasting snatch
Garment effect: x% chance of autocasting snatch
Since autocast effects on the same skill take presidence in the order the items are equipped, this works, as long as you have the garment on before equipping the set. However, since set effects are accounted for first, if you put the garment on last, the set effect will "disable" the autocast (which isn't present to begin with), and then the garment will enable snatch autocast.