@hesh230 No, it shouldn't reset your Heirloom counter because this Heirloom is not attainable with shards at the moment.
The way in which Heirlooms work is that you have a 1 in 500 chance of getting the shards in each pack you open, the more packs you open the higher the chance.
As the Caustic heirloom is currently not attainable with the shards it is therefore exempt from this and is a 'promotional item'. Basically, the event is a 'fast track' to a Heirloom but will cost you a price.
@KelRieverYou are guaranteed to get a heirloom inside of 500 packs. I have tested this with two of my friend who I played with at the start and I roughly got mine at 440 packs, my friend got his at 400 and unfortunately my other friend was roughly around 493-497. It makes sense because we bought packs together and was roughly the same level the whole way through. People do struggle to keep track of it. Since getting my last Heirloom I have taken note of the amount of packs that I have opened.