A couple of thoughts from me.
First, the soil - those mints like a neutral or slightly acidic soil. Though you said nice loamy, I wonder if either the pavers are new, or there is cement holding uprights for the deck, which are leaching lime into the soil & making it too alkaline.
Second, the plants themselves. I think I know the label & if it's the same then I also lost a couple of (different) plants.
It may be that what you bought, were over-rooted and kept alive in nursery/store conditions by frequent watering. Once in the "real world" esp planted out, there could have been a bit of shock set in.
Over-rooting would be typical in terms of little soil left in the pot so in the ground there is no adequate condition for water/air/nutrients.
I'd suggest you test the soil for PH (can get cheapy kits or borrow some gear from someone who has an aquarium - you have to dig down a bit and mix your soil sample with water).
Consider taking the plants back to point of purchase with a complaint - a decent retailer should refund or replace.
I'm not sure what to suggest as an alternative, but I do think it's better if something you either grow from seed, or from small plugs, to give a better chance to acclimatise. Autumn coming, though, remember temps.