How come the inventory on each product variant resets to 0 after a customer purchases?

Hi all, 

I am helping a client manage their Squarespace store, and it looks like each time someone purchases a particular variant of a product, the inventory on that variant resets to 0.

My client has tried switching the inventory to infinite as well, but it still resets to 0 whenever a customer purchases say, a size 9 pair of shoes (size 9 resets to 0 immediately after purchase, even when there is ex. 20 remaining in inventory), causing my client to have to manually re-enter the inventory number on Squarespace after each purchase. 

Any thoughts on what could be causing this? 

As I said, they currently have their inventory set to infinite, but that has no resolved the issue, and "infinite" -> "0" immediately after an item is purchased.