Just like villager trade system, dweller trade has limited number of items each time.
However, by shift-clicking the goods, it displays "×" as if limited items are bought, but once clicking the items bought just now, it will suddenly become the number that will only happen when all cash are used up.
The bug can be avoided by not using shift key to operate.
Here are the pictures about this. Clicking the items of the first picture, the situation will become the second picture.