Even though you're setting the price for a bunch, the plugin will only see it as setting the price for 1 item and then it stores the others for future use.
It's not working the way you seem to be hoping it would or you saw in other servers. It's redundant but you'll have to put up each potion for sale on its own, honestly you should just use trade signs to sell the potions it's much easier.
I hope this answers your question, I'm closing topic since as far as we know, this is not a bug.